Someone's knowledge, experience and actual ability (on any given day, in any given set of circumstances) to recognize and both
properly &
effectively resolve the problem, within a time frame which might be useful in any given set of circumstances, is a subject which has been studied & discussed by any number of different folks.
Presuming the ability to reload ... or identify a magazine-related problem, resolve it and load the weapon to continue normal operation ... and a reasonable expectation of how quickly these things ought to be done by someone is probably going to vary fairly widely. After all, there's the sporting/competition folks, the professionally armed folks (who may, or may not, be "firearms enthusiasts") and the privately armed folks (who may have any variety of interest in such skills, let alone training to learn them and effectively practicing to maintain them).
Which "standard" are
you going to adopt and adhere to when it comes to carrying a pistol as a dedicated defensive weapon?
While I've formed some ideas after having watched a lot of folks do such things on training, practice & qual ranges over the years, to varying degrees of skill, the actual circumstances, awareness and mindset which can occur during an actual "real life" situation may well evoke a response other than anything previously experienced in training, or expected, for that matter.
An example ...
I know of a LE shooting where a fellow was using a popular, well respected and admired plastic framed .45 pistol, loaded with one of the well respected LE-Only lines of hollowpoint ammunition. Unfortunately, as soon as the fellow encountered an armed suspect who fired a weapon, and he brought his own weapon into use, he experienced some feeding stoppages which required time to resolve. Fortunately, another cop was present who was able to engage the armed suspect (hitting the suspect several times), during which time the cop with the malfunctioning .45 was able to avoid getting shot and finally get his gun functioning again.
How qucikly is "quick enough" to resolve a stoppage or other malfunction? Dunno. I can come up with some "acceptable time limits" for training & qual situations, but I can't presume to suggest whether those "times" will prove fast enough for any particular situation or set of circumstances which might befall someone outside the range environment.
I can suggest, however, that having an understanding of how to identify and resolve such problems might be more practical, and perhaps even critical to know at some future time, than ignoring the possibility and not learning how to deal with such things under the watchful eye of an experienced instructor.
If the successful and effective resolution to a magazine-related problem/stoppage requires the availability of another magazine, and someone doesn't have one on them at the time it's necessary, then even having the knowledge and skill might become a moot point.
(Which is one of the reasons I stopped the car and went back into the house to retrieve my spare magazine the other day.
)
Now, how fast is fast enough? Well, if you ever arrive at a definitive answer ... other than "fast enough for whatever the situation may be" ... let us know. I'll pass it along to the other instructors and the folks I train.
It probably won't matter to anyone who doesn't carry a spare magazine, though. Not in time, anyway, should they find themselves in that sort of situation.
Oh yeah, I skipped the part of the original question regarding whether to carry a "full-size compact mag". I've done both. Mostly I've simply carried the stock mags for whatever pistol I was carrying. Depending on the specific model/caliber, and how well it could be expected to function when used with a mag from the next-size-up model, I've sometimes carried a larger spare. Just depends. I can think of the instructor in one of my many armorer classes addressing such a question (which isn't uncommonly asked). He hesitated ever-so-slightly, and then answered, "It should". Note his apparently carefully selected choice of words. I did. So did some other armorers.
I've also seen some instances where one or another full-size mag exhibited erratic feeding in a compact/subcompact.
Just depends. The caliber (recoil force and resulting slide velocity), as well as the age and condition of the springs in the larger mag, can also be factors to consider.
Me? I check and and verify. Often. I won't use larger mags in smaller pistols if anything other than optimal feeding & functioning is always observed. Other folks may suit themselves.