See this Youtube video for a guide of how to use Sticker Builder to create custom sticker parts for Studio 2.0 and Brickhub.org.
Select a PNG image and the size of the sticker. The size is given in LDU's.
As an example. A 1 x 1 Brick is 20 LDU wide and 24 LDU tall, while a plate is only 8 LDU tall.