Its the Epson canvas that is made with a blend of cotton and poly and its water resistant. I produce three sizes between 12" x 15" - 20" x 25". Strangely, its the smaller ones that seem to have more sagging problems. I suspect it may be an issues where the canvas just got too wet when I misted the backside to help tighten it up. This paper is very commonly used, but I just started printing on canvas a few months ago, so thought I see what others were experiencing.
What is appropriate size to begin using cross braces?
That depends on the actual frame itself ... there are several types of stretcher moulding. Easy enough to find out if the frame sagging is causing the issue ... jam something in there and force it open a little bit.
Probably easiest thing would be to re-stretch them. Sagging can also be from just not getting them tight enough to start with.