Hi Ron,
I have been using the Monarch envelope for several years. They aren't as
long as the #10 and will fit any QSL's that I have encountered.
The Monarch size is 37/8" high by 71/2" long and I have never recieved any
QSL's that were cut down to fit.
I hope this is of some help.
cu es gl
Dave K6WRF