I'm sure there are sizing questions on here somewhere, but searching for them yielded lots of not what I wanted. So I'll be a pain and start a new thread.
If I can't find a bike that seems like the perfect (53cm) fit is it better to err slightly too big (54-55cm) or slightly too small (51-52cm)? Or should I just be more patient?
Thanks.

Within a cm of fit, you're not going to find a huge difference that can't be adjusted for via stem and saddle tweaks. However, the one thing that I don't like about having a slightly too big Cross Check is the fact that I have pretty much no crotch clearance on flat feet.

i'd go slightly smaller. it's generally easier to size up a smaller frame (mostly the stem and seatpost), but difficult to impossible to make a larger frame fit smaller. maybe more accurately, a frame can readily if wonkily and oddly, extend to adapt to a longer person, but quickly meets a limit for a shorter person since the adjusting bits are limited by the frame's fixed size.

measure for your legs and then adapt the bike for your monkey arms? just kidding about the monkey arms.

Depends what you're using the bike for. For my touring bike, I was much more comfortable on a bike one size "too big" than all the fit calculators claimed. For a sportier bike, one size "too small" can be adjusted out.

Like mulveyr mentions, what's the intended use of the bicycle?. Also, what do you personally like in a bicycle? Start with the bike shop salesman's "2 inches of top tube clearance" standover fit test as a starting point and adjust from there. Personally, I like a road-style bicycle where my "junk" (can I say that here?) is resting on the top tube when I'm standing over it, and just a little more clearance on an MTB (I don't ride aggressive single track, just fire roads). And when in doubt, I go for the slightly smaller frame sicne it can be adjusted via the seat tube and stem. Your mileage may vary . . . .

I agree. The adjustable items, saddle height and fore-and-aft distance, stem length and angle, and so forth can all easily accommodate a small discrepancy in frame size.
My current vintage ride is perhaps a tad too large, but I have a very comfortable riding position.

Try a different manufacture. All of them build slightly differently and a 51 in one may fit aswell as a 53 in another. Get out and test ride a few bikes.

I "Like" a 51cm with a 53.5cm top tube- but can ride a 49 or a 54 and have all 3 fit me. In fact I do.

The critical bit for me is the top tube length- not the frame size.

It is more of a matter of preference. I pick frames a tad on the small side.
That and it is easier for me to adjust a slightly too small frame than a too large frame. My old touring bike was slightly too large. No matter what I did it was never ideal.
My racing bike is on the smaller end of the scale and comfortable as all heck.

We're probably going to be in the minority here, but slightly too big (1 cm in a 64 vs. 65 tossup) worked for me too. I'd been riding a 62, and Rivendell's sizing guide put me on a 64cm Atlantis. It's great--but a few months later I got a really good deal on a used 65cm Rambouillet (another Rivendell model), and it's probably a little more comfortable.
As another post noted, though, not a huge difference. By old-school standards, many people are on frames that are too small for them anyway.

This is very helpful so far, thanks. My intended use is triathlon and training. Sounds like I'll be looking slightly smaller. All I've been using is top tube clearance as a measure because I don't know better and am new to this. Again, much gracias.