I can get around four yards in "easily" - without having to really wrestle it into the envelope.
Maybe six yards if it's a comparatively light weight fabrics
.
I've gotten up to seven yards in the envelope - but that required major distortion of the envelope and a lot of tape to hold the envelope together. I'm not going to do that anymore - it takes a very long time to fold the fabric and smush it get it in.
I've been told that USPS is getting stricter about how much distortion and tape is allowed on the Flat Rate Envelopes.
I've heard of some people being able to fit nine yards of regular cotton quilting type fabric in a regular size FRE - How is it done? I haven't been able to do it.