In reply to moonchip:
Fat ropes (~11mm) are really for outdoor centers and the like or where extra toughness may be required. Given they're heavy and add little performance wise they're pretty niche these days.
Ropes between 10.5 and maybe 10mm are the workhorse single rope sizes, they're light enough for most climbing and tough enough to last a fair while. You'll probably find the particular weave and surface treatment changes the feel more than the small differences in diameter.
<10mm Singles are still a bit niche, they're aimed at climbers prioritising low weight over longevity. They can be pretty slick and difficult to hold without a suitable belay device. I'd not shy away from a good deal on say a 9.8mm but I'd be wary of the really thin ropes, they're generally pricey and need to be looked after and used carefully.
In reality longevity seems to have as much to do with the sheath weave and the use as it does the diameter. Thin ropes tend to handle and feed better but are harder to hold when abbing or catching a fall so care is needed in matching them with suitable kit. Fatter ropes weigh more. If they're going to get wet and freeze then dry treatment is worth the extra.
For a bog standard getting started again rope something cheap around 60m x 10mm will be easy to use and should last well. Don't get hung up on fractions of a mm, you'd struggle to measure the difference let alone discern it without tools.
jk