a) what's the recommended size for an ESP? (I found inconsistent suggestions on the web, from 1 Mibyte to 200 Mibyte!)The absolute minimum size that d-i will let you use for the ESP is 35MiB, based on parted filesystem options. The minimum size I'd use personally is ~200MiB, but the Wikipedia UEFI article suggests 512MiB, and that's what we offer by default when doing guided partitioning. As Stefan says, it really depends on what you're likely to need to put in the ESP.
IIRC, 512MB is the minimum spec-compliant size for the ESP on a 4k Native drive. The minimum spec-compliant size for 512k sector drives is less, though I don't remember what it is exactly and am not up for spec-diving at the moment...
Honestly, if you're that strapped for space, the ESP isn't the first place I'd compromise on.
--Jared -- Jared Domínguez Infrastructure Software Engineering Dell | Enterprise Solutions Group