When should I upgrade?

I am glad that you mentioned that you recently replaced the front tire, because that does help. It helps immensely. The KX100 is a fine bike, and for a shorter fellow like yourself it would seem to be just about the right size. But, RC is about your size and he rode a 450! I would say much of the choice to move up (and I would move up to a 250f and not a 125) is a skill thing. When you feel comfortable on the bike you are on, and also kind of feel like you could use some more power to go a bit faster, jump a little more, etc. then it is probably time to move up. And that will naturally come with more time on the bike. You will kind of get a sense of when you want to move up, and it may help to try a 250f out and see how you feel (maybe try a friend's or test ride one).

if your just play riding it doesnt matter, whenever your comfortable sitting and handleing a big bike. i would let yourself out grow your bike height wise before anything, which would probably be around 5'7. theres no sense in riding a 250 when you cant touch the ground, if you race though the mini classes are based alot around age so you have to fit into the age and displacement requirement, but learn, the kx100 is a great bike for beginner to advance. the 2 stroke will always make you a better rider, too, coordination and timing wise. it teaches you to shift, effective use the clutch not abuse it and keeping the power to the ground. so when you look for a new bike think 125 or 250, maintance is cheaper and they way more fun.

Haven't we been through this before?

KTM 150 would be perfect

Being a beginning rider, staying on the small bore 2 stroke will be very beneficial to improving your track skills. Learning to carry your momentum through corners is much easier on a small bike that you can manhandle and slam berms with the gas on without it getting away from you. Having to ride your bike more aggressively than other bikes on the track is great practice. Check out the video on this board of Jimmy Decotis on a 125- practice like that will make you a better rider.

my friend rode a kx100 for 2 years and just barely got on a 250f (he's 17 )
but he could haul on that thing! a 125 is a way harder bike to go fast on, but it will improve your riding skill. but so will the kx100 i think. me and my friend switched bikes for 3 laps, and he got SMOKED by me on his 250f. but when i'm on my bike i almost keep up with him, but not quite. pretty much, 4 strokes are easier to ride, and faster. i wish i had bought one instead of a 125, but the 125 has given me lots of good experience, and i can work the clutch far better than any of my 4 stroke riding friends, and it's a lighter bike, and controling it in the air is easier, but much different feeling that a 250f. mine feels almost like whipping a bmx bike, while bigger bikes feel totally different, and more stable to me actually. thats just my 2 cents haha

About the height, you can always get a short seat (really easy with the KTM hardparts) and/or shorten the sub-frame. I mean I'm also 5'4, I have a short seat on my KTM 144 and really the only part that I struggle with is touching the ground on starts but you can always get/make starting blocks for that. I've never noticed it on the track, even through the whoops.