You can change the quality of thumbnails in Windows 7 and make a custom settings. The size can be changed easily by selecting from drop down menu in the extreme right top corner; but, you have to follow a trick to change the quality of the thumbnails.
Regarding size, here is a screenshot of default sized thumbnails:
Click Start button; type “regedit” (without the quotation marks) and hit enter from keyboard:
It may ask for security prompt, just accept it. Registry editor will open.
Find the following string:
Right click on the right sided pane and Create a 32 bit DWORD value called ThumbnailQuality, and set the value between 50 and 100:
99 percent work is done. To make it effective, you can either restart your computer or restart the explorer.
Note: You practically not see any change in quality of thumbnails if you are not using biggest sized thumbnail for preview; but will soon discover it is working by increase in thumbnail loading speed.