Google Chrome's "new tab" page uses relatively small (~140x80) thumbnails.
This, coupled with the page's title is clearly enough to quickly identify most sites. Especially so considering these are the pages you visit most often.
However, consider this point:
In your case, the users will be looking at screenshots from their own websites. Meaning, one website.
So, unlike in Chrome, they will all be quite similar. Same cholor scheme, same header (usually). Imagine all New Tab suggestions would be different pages from the same website. It will be much harder to differentiate one from the other.
So, I think a set of screenshots from one website would require larger thumbnails.