You can give Easy Thumbnails a try.
That's what I'm using atm, it's a free program and seems to work pretty good.
It will give you the same ratio as the big pics as Lemmy and GG was suggesting.
You can go to settings and set max width and height. Also when setting max width and height you can set them to the same max and it will still fix them going by ratio of regular pic, just doesn't go past the max either way.
You can do a whole folder at one time or just 1 or 2 in the folder and it's a fast program. It also shows you a preview of the thumb going by your settings before making the pic.
If you want to give it a try, this link should take you to it.
http://www.download.com/3120-20_4-0....ils&tg=dl-2001
Hope that helps,