You can see the solution of each levels including the location of each tortoises and secret levels in the following video :


Timecode of levels and achievements
0:26 Tutoriel
2:20 Level 1 Seaside
5:22 Level 2 Desert
9:13 Level 3 Campsite
11:29 Level 4 Seasons
14:48 Level 5 City
17:38 Level 6 Themepark
20:27 Level 7 Space
24:00 Level 8 Halloween
26:50 Level 9 Golf
29:11 Level 10 Sakura
31:21 Level 11 Deepsea
33:58 Level 12 Medieval
36:30 Level 13 Farm
39:30 Level 14 Easter Isle
43:45 Level 15 Jungle
47:19 Level 16 Time Machine
51:48 Level 17 Mine
55:16 Level 18 Ants View
1:00:42Level 19 Swamp
Achievements :
1:05:29 A Sizeable Victory! : Complete the main levels of Sizeable.
1:05:40 Level 20 Northpole
Achievement :
1:11:00 Shellebration ! : Find every tortoise and unlock the secret levels.
1:11:15 Secret level 1
1:12:51 Main Menu
Achievement :
1:14:27 From the Beginning! : Find the secret level hidden in the main menu.
1:14:40 Secret level 2
1:17:28 Level 9 Golf
Achievement :
1:19:15 Hole in One! Unlock the secret golf levels
1:19:28 Secret level 3 Golf - Halloween
1:22:35 Secret level 4 Golf - City

Developer plan to add free updates, so you will found it here.

Added a new small secret level + achievement

0:30 Level 15 Jungle
Achievement :
2:37 Welcome To The Jungle! : Unlock the secret Jungle golf level!
2:48 Secret level 5 Golf - Jungle

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The update is called: Wonders of the World! It include 5 new levels, more turtles, Achievements,...

Achievement :
The 5 Wonders of Sizeable! : Unlock all Wonders of the World levels!
1:06 Level 1 Stonehenge

• 2:24 Tortoise

3:09 Level 2 Tower of Pisa

• 6:24 Tortoise

6:45 Level 3 Gizeh

• 7:36 Tortoise

9:40 Level 4 Taj Mahal

• 11:24 Tortoise

11:46 Level 5 Pharos

• 12:34 Tortoise

Achievement :
Faster than Ero?!
THIS IS AN OPTIONAL ACHIEVEMENT! Reset your game and go through Sizeable's main levels + wonders of the world in 30min or less! One sitting, but no tortoises or secrets needed!

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Added a new small secret level + achievement

0:37 Level 6 City
0:55 New secret level 6 : Sewers
Achievement :
TMNP! : Find every hidden Ninja Pippin in the Sewers level!

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The update is called: Among the Stars! It include 4 new levels + 1 secret level, more turtles, Achievement,...

0:37 Level 1 Observatory

• 1:25 Tortoise

2:52 Level 2 Mars

• 4:08 Tortoise

5:54 Level 3 Spacegate

• 6:11 Tortoise

8:44 Level 4 Planet P1PP1N

• 10:00 Tortoise

11:11 Level Tower of Pisa
Achievement
11:57 Aquaduct to Rome! : Unlock the secret Colosseum level!
12:04 Secret level Colosseum

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Added a new small secret level

0:45 Level 28 Mars
1:03 New secret level 7 : Black Hole

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The update is called: Once upon a time! It include 4 new levels + 1 achievement, more turtles

0:30 Level 1 3 Piglets

• 0:46 Tortoise

1:57 Level 2 Rapunzel

• 2:13 Tortoise

3:21 Level 3 Cinderella

• 3:48 Tortoise

4:53 Level 4 Wonderland

• 6:47 Tortoise

Achievement :
6:51 Cheshire : Find the Cheshire cat!

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Added a new small secret level Oz (+ Fixed a bug in the main menu that caused a problem that a secret level couldn't be unlocked)

0:55 Level 14 Farm
1:41 New secret level 5 : Oz

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Added a new small secret level + achievement
Halloween Level is the 1st of 4 Holiday Levels, these levels will also only be available during the real holiday period.

1:34 Cabin Fever!
Find the special halloween level that is available from 28th of October - 3th of November!
3:06 Tortoise

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Added a new small level + achievement
New christmas level! This level will always be available from 21th of December until the 31th of December!

0:26 Ho Ho Ho!
Find the special christmas level that is available from 21th of December - 31th of December!

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Valentines Day Level + Secret Level - How to unlock Seasonal Levels - Diorama

0:38 Valentines Level
Achievements :
<3 : Find the special Valentines level that is available from 11th of February - 17th of February!
1:40 Prrreciousss! : It's mine, my own!

1:58 How to unlock the secret level
2:58 Northpole level
4:46 Secret level : Golf - Northpole (no turtle, like in every secret level)

6:43 How to unlock seasonal levels ?

7:44 Diorama mode

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Added a new small level + 2 achievements
New Easter level! This level will always be available from the 1st of April - 12th of April!

Achievements :
0:45 Happy Easter! : Find the special Easter level that is available from the 1st of April - 12th of April!
4:58 Eggcelent! : Crack open all 17 eggs in the Easter level!

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The update is called: Community Matters! It i new levels + 4 tortoises + 1 achievement

Note from the dev : "IMPORTANT: If the new level doesn't unlock automaticly, you will have to replay the "Wonderland" level in order to unlock the new "Giant" level! Sorry about this! This will be fixed next week!"
1:04 Wonderland Level
3:00 Giant Level
3:21 Turtle
6:10 Sky Island Level
7:13 Turtle
10:07 Arcade Level
10:35 Turtle
14:34 Sizeable Level
16:47 Turtle

23:52 New homescreen : This new home screen is bugged when I made this video, the 3rd pillar (pillar n°1) is not being able to be found.

Achievement :
14:28 All Done! : Unlock every normal level in Sizeable!

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The dev have added the level "Lantern City" in celebration of 30 Birds

0:00 Lantern City level
1:21 Turtle

You can follow me on my YouTube Channel here :

Fredericma45 Gaming