Whichever ukulele you buy eventually, make sure it is properly set up. We have just been talking about it
here.
Barry Maz
reviewed the ACU-SUS just a couple weeks ago, which is a surprisingly inexpensive, massive mahogany concert-size uke for just £80 (and it is offered by a UK store). The real downside, I feel, are the GHS strings it comes with, but a few bucks (or quids) buy you better strings, and from the sound of it, it's a good deal for the price.
The Stagg UC-80-S is also a full mahogany concert ukulele that I paid only €99 for, including Aquila strings, but it needed a setup and it seems to cost normally more than this and apparently often comes with GHS strings (this may have changed recently -- I bought it from an online music store and it came in original packaging, with Aquilas). The high action aside, it is a good instrument.
As for the size, eh ... chances are you will end up with at least one of the common sizes (soprano, concert, tenor). It didn't even take me a month for that! Personally, I seem to enjoy the soprano and the tenor the most, with little need for the inbetween size, but this is very subjective and my opinion may well change in time, too. Plus, there's more to this than just the sizes. The necks are not always the same, and neither is the space between the strings.
I'd say just buy one and then use your experiences with it to refine the next purchase! Just mind the setup, which I feel is perhaps more crucial than the size, for the first ukulele.