What are the other decks of similar size (or smaller) to the PMD-661 that also have a coax digital S/PDIF input?
It's unfortunate the manufacturers aren't listening to tapers suggestions about including this in the feature set of their products. Coax SPDIF is usually one of the first features tapers ask for in a small low priced digital recorder. However, the bottom line is that there simply aren't many low cost options...those that are available have already been mentioned. Some added thoughts to what's already been mentioned...
If all you want to do is use it for digital is SPDIF input recording, the MTII might be your best option, if only because it's very low cost and fulfills the criteria you've mentioned. It tends to work reasonably well if all you want to use it for is a bitbucket. Otherwise, the deck can be a pain in the ass and has features that will probably cause you pain if you try to use it for anything other than a bitbucket. Personally, I'd avoid it if you're also gonna be using the other inputs because it's just too frustrating losing recordings because of its quirkiness.
Next in line is the Sony D50 at $450, but that has an optical digital input not coax, so that doesn't work for you.
Then there's the 661 that has been mentioned. Great little unit. Be aware that the XLR inputs on the 661 need to be balanced.
Beyond that, I don't think there are any new decks in the sub $700 price point that offer SPDIF coax input.
Used, the HD-P2 has been mentioned. I'd also mention the Marantz PMD-671...and there have been some good deals on used 671's in the yard sale lately. Neither of these meet your size criteria though.
Finally, the R-44 is small-ish (not stealthy small though), but used prices are more than $700.