You're going to have a hard time believing this but after buying a Nashbar touring frameset, I couldn't make up my mind which parts I wanted to buy so I ended up with two cranksets, two front derailleurs, two handlebars, two stems, two set of brakes, so on and so forth.

So, I decided instead of selling things at a loss on eBay, I could always just make another build. The Nashbar touring frame I have is 54cm. I followed instructions on the Colorado Cyclist website to measure my inseam. It's 83cm or 32.6 inches. The sizing calculator and even the Rivendell sizing chart say I take a 56cm. However, when I put wheels on the frameset with 700x45 tires, the top tube came close enough to my crotch. I doubt I'd really want a 56cm. I also looked at an LongHaulTrucker group spreadsheet with frame sizes people chose. And for 5'8" (my height), most people chose a 54cm frame.

Anyway, Nashbar has a cyclocross frame but they don't have a 52cm. I kind of wanted a 52 thinking if I need to put my foot on the ground in rough challenging trails, the top tube wouldn't hit my crotch. I have a choice between a 50cm or 54cm. The 54cm looks like it's really stretched out with a 56cm effective top tube. The 50cm seems tempting like hell because it looks like I'd be nimble like a madman. I mean just the idea of it would puts a smile inside my head just thinking about it.

So, what do you think? Is it worth it? Should I go ahead and get the 50cm cyclocross frame or just get the 52cm touring frame? This bike would be more of a city bike/trails bike than anything. The 54cm touring frame would be my real touring bike.

Are those the only two frames you are willing to consider as there are other options?

You may find a 50cm cross bike frame to be too small. I have a 50cm Bianchi Castro Valley (same geometry I think as their Volpe). I am 5'6" on a good day with a relatively short inseam - probably about 3" less than yours. The 50cm fits me fine, note I've never used tires bigger than 35mm studded on it. Or at least you will have plenty of seat post showing - useful for mounting large saddlebags. A longer stem can help some with handlebar reach, but it'll still be a compromise. I'd keep looking for a frame that is the correct size, unless you need to build that bike NOW.

45mm tires are pretty big - do you really need something that wide?

Actually, at this point, I'm willing to consider other frames. I've read some comments on the Nashbar cyclocross frame that were kind of a turn-off.

Well, let's say I tried the 45mm tires as an experiment and now believe they're just too big. I think somewhere around 37, 38 or 40mm may be in my comfortable zone. I know 35mm is a tad small when going over railroad tracks or the many potholes around here.

tire size can be personal preference, I use 28s on my cross bike (and toured on 28s for years) but was and am nearly always on pavement. I just prefer a lighter tire that makes the bike seem more sprightly, I have 40s (1.5) on my commuter mtn bike which I admit is nice on Montreal roads as you dont have to be so attentive and careful of your line over and through rough stuff, not to mention gravel.
Oh, at a bit more than 5'10", I ride a 54cm Tricross and it fits me very well. I am definatly in the "preference for a slightly smaller" frame size camp (but could be my leg/torso ratio influencing that)

Cross is as much about getting on and off the bike and carrying it, running and jumping barriers

as it is riding it.

so that is why they get sized smaller and made lighter..

I have a 32" inseam and I ride a 58Cm frame with a level top tube (not sloping).
Of course, the top tube is close to my crotch while straddling it but it never seems to be an issue.

lol, I went to a Cannondale dealer in Chicoutimi (Saguenay), Quebec, Canada and asked to see the Trek 520. The guy's expression was pretty funny. They did not sell Trek bicycles. Anyway, I ended up at the Ultraviolet store in the same area and they measured my leg height if you know what I mean. I'm 83.5cm (at 5'8") and they opened a book showing I could use either the Trek 520 in 51cm or 54cm. I sat on the 51cm and the feeling was amazing. It looked like the perfect smallest frame I could use for better maneuvrability. I'm used to flat bars but got a little feel for what it would be like to ride with drop bars. They're different than in the 1970's and 80's because you can now rest your hands just above the brake levers. I still prefer flat bars though.

This means I'm safe to buy a smaller frame.

I'd also like to note that some sizing calculators or charts had me at 56cm. I guess 51cm or rather 52cm is two sizes down. So, I can't completely agree with the charts/calculators.

If possible, try to get on the frame size you're interestid in for a couple of hours riding.
I only get sores after about an hour on my too small, old mountainbike. For shorter rides it feels fast and light... so be careful when buying a small frame

salut, I am 5'10"-11' and I fit great on a 54cm Tricross, so at 5'8" I could see a 51 or 52 working well. I also find I dont agree with charts/calculators.