Race XC and MTB 100s. I have been using 2.25 f/r. Was having more front washouts (cornering) than I like, so this year I’m switching to 2.35 front and 2.25 rear hopefully to correct this. Obviously tire choice and handling are significant factors that play as big or bigger roles than just tire size. The myth of smaller tire size means faster rolling resistance has been debunked. In Michigan, the standard is 2.25 -2.35 for XC racing. Haven’t seen much of anyone going smaller or larger. All that being said, tire compound and tread pattern is likely to have a greater affect on how the tire feels.
I’d second the comment about tire pattern being more important than size. I was washing out regularly on 2.2 Race Kings a couple years ago. I switched to 2.2 cross-kings (not exactly aggressive either) on the front and it pretty much ended the problem. I have since moved to 2.25 rocket rons on the XC and 2.4s on the trail bike and things are even more stable in the upper midwest dirt. Larger volume tires seem to be more important for areas with chunkier, rockier terrain. Bentonville definitely felt like a bigger (and thicker) tire would be beneficial. The calculus on a hardtail might be different, but that’s not my game.
Our local races are all hardpack trails with very little loose stuff on top. I’ve been happy with 2.2 Fast Track front and rear. This year I’m switching to Maxxis. I’ve been really happy with a 2.35 Ardent race up front and 2.2 in back. The Aspen intrigues me as well and it looks like they have similar width offerings. Pattern and profile have more effect in my mind than does the width. Your trail conditions also dictate what patterns will work best. For me, low profile, easy rolling center knobs and some decent shoulder knobs are the ticket.
Last year I ran 2.1 Maxxis Ikons front a rear with various tire pressures dependent on the race course and it suited my riding style fine on the hardtail, on my full squish I made a few changes and run Front Maxxis Rekon 2.25 / rear Ikon 2.2. So far this setup has done well on a mix of courses and piled up quite a bit of new PR’s.
Optimal pressure is very important, but can only be properly set after knowing the first two. If by pressure you mean having it optimized, it is probably #1 or #2. My current tires aren’t fast by any means since they are for more aggressive riding, but the big 2.4 on a 29 does not want to be above 25 PSI for me, especially if the rocks are damp at my local trail and with the EXO sidewalls can handle riding rocky stuff at 22 PSI. For my single speed, since I ride it rigid, I ride at 18 PSI on a 2.3 front to be a little more forgiving and 20 psi rear, also 2.3. I usually try to run a semi slick on the rear when I can get away with it, but have gotten burned not changing it out when conditions on race day were damp and had to walk up a few sections.
When on my 26er, it used to be 25 psi front, 28 rear. One of the biggest advantages for 29er wheels is that bigger contact patch, made even bigger since you can ride at lower pressure.
Everything you listed will be different for every person and what works for them, with full suspension MTB’s the suspension set up will also factor in quite a bit.
What tread/comp/width will be deeply dependent on your personal riding style and the types of trail substrate, rocky, sandy, hard pack etc. that you find at each race location.
I have ran anywhere from 18psi to 29psi at various places, Ikons and their pattern work great for most of the trails we have in Texas, Because of the way I ride I tend to benefit from a more aggressive tread on the front.