Working with Limitations

You're the only one who can answer that question really.

My old XC bike was a medium but according to all the charts etc, I should have gone for a large. I tried both but preferred how the medium felt through tight singletrack.

Have a play about with the saddle, try moving it back a little bit. Remember though, by moving back the saddle, the front will be prone to lifting on climbs and front end grip will be reduced.

A longer stem can stretch you out a bit, but you dont want all your weight too far forward either, and if you get too stretched (or too cramped for that matter) you will probably start to get sore shoulders, back etc.

It can be a bit of a balancing act sometimes, getting just the right position on the bike, but once you have it, you'll know.

Oh, if you are 6ft4 and riding a small frame, forget all the advice above 'cos you're screwed.

critical dimension is the top tube (cockpit length) This, as stated, can be quite personal.

ride the bike. Get a longer seatpost if needed. If you feel yourself riding too far back on the saddle, then slide it back a bit on the rails or get a bigger offset post. Depending on the length of your stem atm, you could think about a longer stem but on't go too far, handling is affected. Again this is personal. I like shorter stems but I suppose industry standard for XC is 100mm. You'd be surprised how much difference a couple of centimeters in the stem length makes so try out different options, perhaps borrow old stems from your mates or a LBS. Most modern stems swap out in seconds.