Yes Another One!

Go with a 125 2 stroke. The KX100 and any 85 you will both feel cramped on, especially your bro.

125 isn't too big. I'm smaller than you by 3" and 25lbs. On my CR and now my YZ we put softer springs and shaved the seat about an inch. You're not going to be able to touch flat footed, but can at least get a foot down on one side. I can barely touch w/both of my toes. KX100 isn't too good for the dunes a 125 really isn't either, but it can be done. Once the new springs break in, it gets a little better.

No problem.
The YZs are one of the taller ones. It does squat a little once I sit on it. Also, it's kinda sitting a little higher than the spot I'm standing, probably 2-3" higher. If you have the suspension set up correctly it will squat a little when you sit on it. Like I said I can touch barely with my toes on both sides or I can slide over on the seat and touch almost flat footed w/one foot.
When I got it I couldn't touch at all w/the stock springs. Same way with my '01 CR125.
They all lower a little when you sit on them. Remember that if you are at a dealer sititing on bikes the suspension is not broke in, the springs do break in a little bit after riding. Stock suspension is for a rider around 165-175lbs. When I ride my husband's YZ250 the suspension does not move. Adjustments can be made for us short people/lightweights.

RC is 5'6" and around 155lbs (I think) rides a 450
Ryan Villopoto is 5'5" and 140? rides a 250F
There was an article a couple of months ago in Dirt Rider on quite a few of the top pros and it gave all their specs. Also listed the mods they had done to their bikes: pegs, frames, suspension, etc.

I race w/a girl that is 14 5'2" 100lbs and rides an 85 and a CRF250.