Building you're own stretched canvas is so simple, and SOOOOOO much cheaper than buying a ready made one from the store, especially one this large. the finished dimensions are 48" by 48". 

What you need:
Mitre saw (chop saw)
Staple gun and staples
60" of 60" wide canvas (I get mine from Joanns. it is in their duck cloth section and costs $9.99 a yard, use a 50% coupon) 
(3) 2x2x8's
Electric drill

start by cutting 2 of your 2x2's in half, so 48". So you will have 4 48" pieces.