Almost all USB drives will work if prepared properly but it is always good advice to use a known brand, size/capacity is determined by how much you wish to record, hold on the drive &/or use timeshift.
On our main system, we have a 1TB Samsung external USB drive powered via the box USB port although you are better advised to use an external power source if using an external HDD, on other systems around the house I use whatever USB drive is to hand.
We hardly ever record or use timeshift so the drive remains empty most of the time although we do sometimes d/l films/series/boxsets to the drive.
On the other Engima2 systems we have I use any USB drive I have to hand, often 16gb - 32gb without any problems although again recording/timeshift on these is minimal & cleared/deleted from the drive once watched.
Most decent IPTV providers have a 7 day catchup service which removes a lot of the need to record so really USB drive capacity comes down to how often you use timeshift & whether you clear timeshift once used.