A 10x5 is a fantastic all rounder, holds 2 bulk bags nicely, 3 ibcs fit if you turn the tailgate round the 'wrong' way, takes a couple of ton of loose sand etc. Small and narrow enough to run around with most places but a genuinely useful capacity. I had a 10x5 ifor tipper with full mesh kit, that would take a fair bit of gear! Currently have a normal flatbed one, one day I'll get round to chopping it in against another tipper.
If you fit a pair of flexible rubber marker lights on the back corners that brings it out to about 6ft, so it's easier to see in the mirrors but they will bend out the way if you misjudge a gateway....
As I've said before though, if I could only have 1 trailer it would be a 14x 6 flatbed. Surprisingly maneuverable behind a relatively small tow vehicle ('proper' 110, older disco, isuzu trooper) but big enough to take a car or tractor and implement etc