Just bought an used xs specialized sirrus sport in great condition. I am 5'8 and just realized that is too small. Just wondering if there anything that I could to do to make it fit me? Got it today so I haven't try anything yet
I personally have this problem as well, but I need an XL or larger frame and most common ones are Medium or Large.
You can fit a longer seatpost, but they stop at ~400mm with a very few at 450mm long. The danger is that a long seatpost gives more leverage, and I've bent 4 seatposts in ~30 years of adult riding.
Also, trying to get the most height out of a seatpost can lead to frame damage. I had a post at the minimum insert line, and the frame cracked around the seatpost clamp because it was not in the frame enough.
Effective length of top tube can't be changed easily - you can increase the stem length but that has follow-on effects on steering. Personally I have a bad back and a "normal proportion" bike is too long and uncomfortable for me.
If you're talking about UCI compliant frames, then the wheelbase stays the same as height increases, leading to tall bikes that are "lifty" in the front on climbs. Bikes that aren't constrained by UCI limits can be a bit longer, giving you a more stable platform at speed and a better climber. Its impossible to change the wheelbase on a bike without welding.