When I used to work in a bike shop a few moons ago....
It wasn't popular but a size 10-12 jeans is a large, 12-14 jeans is an xl and so on.
Jackets are much more difficult. Same sizing but add an x if she is busty, much easier to bring in a waist than go up a chest size. (note: really busty women tend to fit mens gear better.)
IF your pillion is easily offended by this system use the euro markers. (same sizes but making something euro makes it more palatable.
Leather fits differently to synthetics it tends to be more form fitting but this is because it is much more forgiving. I would recommend always adding an x, failure to do so normally results in either a dieting or a pouting pillion.
For either you want to buy the smallest that will accommodate the largest part, for me (when I wear ladies gear) it needs to accommodate a longer than normal spine and arms and thicker than normal neck.
Going to big in the waist to accommodate this is not a problem get the gear damp a couple of times and it shrinks where its loose and stretches where it is tight.
Just to borrow for a trial run doesn't matter how big it is as long as the armor is in roughly the right area.
I am a 5-7xl depending on cut. Plenty of maidens have borrowed my gear.
Getting an approximate helmet size is easy.
Tape measure around the head at the most bulbous parts front and back.
Then looking down from the top:
Asian brands are fairly square.
HJC and American brands are fairly oblong.
Euro brands are fairly round.
Only the pillion can decide how perfect a fit it needs to be for first ride.
If you are friendly with a particular bike shop most have some older helmets they lend out for test rides and explaining she will need to buy gear if she enjoys her self should net you one.
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