I think this is going to come down to what suits the individual, since some people can't even get used to their Optivisors and eventually sell them on!
There are some previous threads on planetFigure with various recommendations but I can't find the one I was thinking of. Anyway, I don't think there's a consensus per se but too high a magnification is often mentioned as a problem. The #5 seems to be the commonest choice, which gives magnification of 2.5X and the price to aim for in 2010 was around US$30-40.
You might also consider getting some cheap reading glasses instead. Some people use these instead of a proper magnifier and just that little bit of help is enough for them to be able to paint smaller with relative ease and they're more comfortable to wear for long periods. If you already wear glasses, consider getting a pair specifically made to allow close focussing; I wear glasses under-adjusted for distance vision for this purpose specifically, although I have a magnifier also.
Regardless of what you use, it'll take time to adjust to it. I'd recommend you start light (short sessions) and work up, but give whatever it is a really fair shake at the end of the day - nearly anyone can adapt if they just stick it out.
Einion