Good question.

Because of sven co-op's huge ammount of improvements on the engine limits (e.g bigger maps, all textures can be 1024x1024 and a lot of 70k polygon anime playermodels) spray is no expection.
It isnt a big of a deal, but sprays in sven co-op are approximately 15% larger then in original half-life (from 12288 to 14336 surface limit)

The only way to make a spray larger is using Wally, you can edit any texture including your spray and make it how big as you please, as long as you dont go over the limits.

So lets get started!

Heres a very useful chart by our great cat The303


















As you can see, were aiming for the green ones, which are supported by sven
You cant go any higher then that, unless they update the surface limit

1. Download wally texture editor (Be sure to have WinRar or 7zip installed)
Link 1[valvedev.info]
Link 2[gamebanana.com]
2. Extract anywhere you want
3. Launch the Wally.exe
(It should look like this)








4. Press File > New, and be sure to select Half-Life Package [wad3](.wad)





















(This window should pop up)
















5. Change the size of your image from any image editing software and copy it

















6. Paste it as new, and name the texture name "{LOGO"
































(Note: Sorry, only { works, no funny water textures)
7. Save it, and then name it tempdecal.wad


















8. Copy\Drag your custom tempdecal and paste it to
C:\Program Files (x86)\Steam\steamapps\common\Sven Co-op\svencoop













9. Check the mark "Read only" on file options





















10. Launch the game and see the beauty!
Left - Using our own method
Right - Using HLtagconverter (I kinda ♥♥♥♥♥♥ it up :P)
As you can see, the left spray is about 15% bigger then the image on the right
Go wild!