For $2 you can buy this digital PDF file of four different thumbnail sheets. Each sheet has a different ratio for thumbnails. The pdf's are sized to fit standard printer paper 8.5" x 11" so the thumbnails are ready to sketch over.  The image shown is from one of my moleskine sketchbooks and is not one of the pdf's pages in this download, however it demonstrates how useful a thumbnail sheet can be when sketching. 
 
Four Ratios and # of windows per page:
1 x 1 has 12 hand traced thumbnail windows
1 x 2 has 12 hand traced thumbnail windows
1.5 x 2 has 12 hand traced thumbnail windows
2 x 2.5 has 9 hand traced thumbnail windows
 
Tips:
Print in Black and White to save color ink and Increase the brightness/contrast to get rid of some of the tinting in the file. 
 
The copies may vary in size based on your printer settings. The good news is that the ratio of H x W of the thumbnails will change together in scale if printed smaller or larger than the original so they'll still work for your design purposes!
 
If you print two copies of each page you'll have 90 thumbnail windows total. A great exercise is to complete 90 windows in 90 minutes! I call that exercise Ninety Nails!
 
Once downloaded you can print as many copies of the files as you wish. Buying the pdf is a small way to save time not making your own thumbnail sheets in these sizes while supporting my teaching. If you have any questions about the pdf or how to purchase please let me know. Make sure you're signed up for my email list to be notified of upcoming learning opportunities with me.