YMMV on this, but here's my opinion on the questions you asked:
Bigger is always better, provided you can afford it and power it. 6 inch jointer are going to be the most common and therefore the cheapest on the used market. If the majority of what youd be jointing is 6 or less inches wide, you can easily get by with a smaller jointer. That said, if you get a good deal on an 8 inch jointer, and have 220 to power it, you'll never regret the extra capacity. Personally, I'd avoid the 4 inchers. Too small for most general work. Admittedly, if you worked near-exclusively on smaller pieces, say you made models or something, you could get by pretty well with a 4 inch
Internal vs external shouldn't matter too much, you just really want to make sure what you're looking at is an induction motor, not universal, and preferably is easily replaceable if needed. Most are, save for those in benchtop models.
Dont worry too much about blades, most of the sizes are pretty standard and are easy to find.
As far as what to look for goes, personally I'd go for a 6 or 8 inch model, depending on your price range, with the longest bet length you can find, preferable solid cast iron, with a motor ideally in the 3/4-1hp range. I've got a Grizzly g0452 as my machine, and that's what I'm basing my recommendations on, build wise