Near or colliding? Regardless here a clue for what you would use for shrinking:
image_xscale = 0.5;
image_yscale = 0.5;
Play around with these variables, change the values around and you'll get what to do with them. (try image x_scale += 0.1)
As for distance,
point_distance(x,y,x2,y2);
is good for finding distance between 2 points. You can search this function in the gamemaker help for more infomation.
This is all assuming you are using GML code, if you are using the visual editor, you can add a code segment and paste these codes in. They can be used interchangeably!