I’m considering getting a pair of Shimano MT91 boots for winter riding. Can anyone who has a pair comment on the sizing? A review comment on CRC says “I am a 44.5 in most shoes and the 46 fits me perfectly” which make me wary of ordering my usual size.

That review sounds about right. I go one size up for Shimano shoes/boots and then I allowed a bit more when buying MT90s so I could get thicker (winter) socks on.