Can I just order the longest (300) since I'll most likely be cutting it to fit anyways? Thanks.
That's what I would do. You can always cut it shorter, but stretching it will be hard as hell
Bike makers usually sell the bike with the tube cut too short, in my opinion. Getting shorter tubes limits how you can adjust the bike to fit you. You may not need the longest length, but it would probably be good to give yourself plenty of room.
My cannondale is a 56cm and has a 15.5cm headtube, integrated headset and the steerer is 210mm. I have zero spacers. I dont know of any bike that would need a 170 or 200mm steerer unless is was smaller then a 52 or so.
Maybe they were trying to b convenient for people with small frames?
That'd only make sense if the length was exactly what was needed for a given frame--and there are many different lengths required. It takes the same amount of effort no matter how much you gotta cut off.