This is a standard ad block. It is 120 x 240 in size and makes a reasonably nice display for those who want to showcase their product in a small space.
This is a 250 by 250 ad block. It is perfect for something where you’d like a better and crisper picture but perhaps not a lot of text.
This is a half page ad block. It is typically used for larger items or for items that require a good deal of explanation. The size is approximately 300 pixels x 600 pixels