I don't have a SmartQ, so I can't give much input on most of these, but if you're considering the SmartQ a plausible option, you should probably also consider the N800; with the N800, you save $50 or more (vs. the N810) by skipping the keyboard, get a D-pad on the face of the device where you can always use it, and get the full-capacity SD slots...

The usual route with N800s (before the N810, and for those still using them) was a bluetooth GPS, eliminating the cable issue. Size of standalone GPSes, whether BT or GPS, ranges from a large thumb drive up, but the common BT ones typically are around 1.5" x 2" and 0.8" to 1" thick; obviously, the size correlates with battery life. USB GPSes are also usable. All that should apply equally to the SmartQ 5.

As for functioning "like an N810", an external GPS should get better results, as the N810's GPS has a very bad reputation for slowness getting an initial fix. FWIW, of my two BT GPSes, one is about the same as my N810, the other one's somewhat faster, but none of the 3 are as slow as the N810 is generally reported to be. But given the level of complaints online, it's possible my N810 is just a reverse lemon.