First thumbnail bigger than other

You could try using CSS to change the size of the thumbnail. Something like:

img.wpp-thumbnail {
  width: "XXpx";
  border: none;
}

… where XXpx is the size in pixels you want to use.

Yes, but I want only the first one to be Big, and the other smaller.

Alright, then try:

img.wpp-thumbnail:first-child {
  width: "XXpx";
  border: none;
}

I need to add this in wpp.css ?

Yes, or in your theme’s stylesheet. Either way should be fine.

I doesn’t work. It change all the thumbnails.:(