Your designs are great.
I know it what you mean. It can be frustrating at times.
I usually like to make certain designs bigger for that reason.
Heres a chart that Zazzle has for guidelines many people follow.
I don't know if they have all the products there yet.
http://zazzle.custhelp.com/app/answers/detail/a_id/85But if you are deciding to use them on other products, I would go bigger for multipurpose use. Also set your resolution higher so get the best results.
Use one of these image formats:
JPEG
The JPEG format supports millions of colors, which allows for great design flexibility.
The JPEG format compresses the image information, keeping file sizes small and reducing upload times.
Most digital cameras can save images in JPEG format.
PNG
PNG is a completely lossless compression format.
Most graphic design programs (e.g. Photoshop) can create PNG images.
The PNG format includes full alpha transparency support!
PDF and Adobe Illustrator (AI)