There's a some unanswered variables here.
- Is this for an artwork already created and has been removed from it's original frame?
- An original oil/acrylic painting, or a digitally printed image?
- If this is the case, what was the depth of the original frame?
- Is this for an artwork that has yet to be created?
- Is this for preparing a stretched canvas for painting?
- Is this for stretching a digitally printed image that has not been printed yet?
- What is the desired frame depth of the artwork or print?
- Will the finished item be framed or displayed as canvas only?
These are some of several considerations depending on the specific situation.
In general you'll want a frame that won't be too flexible at those spans.
And you'll also want to add enough support structure to aid in preventing bending/bowing when stretching the canvas over the frame.
Also, (if you haven't already) it would be wise to look at the differences between stretcher and strainer frames.
Each has pros and cons.
Figure out what advantages and disadvantages each would have for a stretched canvas depending on your answers to the questions above.
If I want a fairly shallow depth finished canvas then something similar in dimensions to Larson Juhl #26204 would work.
Strainer Stock Stretcher Unfinished 1 5/8" - Buy Online! Larson-Juhl, industry leader in the manufacturing and distribution of timeless custom picture frames.
www.larsonjuhl.com
Medium depth but wider would offer a stiffer frame, such as Larson Juhl #26206
Strainer Stock Stretcher Unfinished 2 1/4" - Buy Online! Larson-Juhl, industry leader in the manufacturing and distribution of timeless custom picture frames.
www.larsonjuhl.com
If more depth is required (usually best with "gallery wrap" style canvas) then I would use something like #6013
Strainer Stock Stretcher Unfinished 1 1/8" - Buy Online! Larson-Juhl, industry leader in the manufacturing and distribution of timeless custom picture frames.
www.larsonjuhl.com
Again, add plenty of corner supports and cross braces, regardless of what frame shape you choose.