How big is the retail market for picture frames in the US and
worldwide? A typical company serving this market is Fast Frame.

(I have an idea for a product to sell to these businesses that might
increase their sales, or I might open my own frame store with this
added concept.)

This article publidhed December 10, 2001 may interest you:
  
“..And as far as they're concerned, business is just fine. FastFrame
is in the business of providing expensive frames for pictures.
Business has grown tougher for independents because of the economic
doldrums and competition from consumer-attractive chain stores such as
Michaels Stores Inc. (Nasdaq: MIKE), said William Parker, a board
member with the Professional Picture Framers Association. Though sales
in the $2 billion to $4 billion annual retail frame industry are down
8 percent this year nationally, Parker's business is up 17 percent
from last year, he said.”
Atlanta Business Journal
http://atlanta.bizjournals.com/atlanta/stories/2001/12/10/story6.html

The size of the picture frame market varies, but is typically 8x10 or 5x7. ha ha