Have you tried sitting on one? How well do you ride the 150? Do you just trail ride or do you clear 50' and bigger doubles on it with ease? Do you race?
You won't know until you try and if all you do is trail ride and putt around on the 150 then don't try the 250. 5'6" is not too small for a 250 but how much do you weigh? 250's with the auto decompression aren't hard to start if you know the right procedure to get it slightly past TDC before you kick.
See if someone has one that you can try to sit on and start but again, don't try if all you do is putt around on the 150.