See the other thread inb this board "mirror sizing" where I explained the math.
To move a 5mm beam (X mirror doesnt care about the shape of your beam, the Y mirror does...) through a 60 degree cone, your MINIMUM X mirror width is 10mm, assuming 45 degree AOI. Note this is 10mm USABLE - Many chinese mirrors will have chamfers as you described, or a chip margin, so the mirror might need to really be 10.5-11mm wide to get 10mm usable space...
Sincerely,
Ryan Smith
ScannerMAX Mechanical Engineer
ryan {at} scannermax.com