max ID = not too big
(she PMed me too, and is too busy building this wondrous topcoil to come back over here and say "nevermind" )
In case anybody else is wondering tho . .. . looks to me (and my calipers) like there's 5mm gap between top of +post grub screw and chamber ceiling (bottom of top of topcap).
• A 3mm ID would be too big, IMHO. With 24g wire (.5mm wire), that'd give you a coil OD of 4mm .. . so, that's just crazy tight and begging for no-no touching.
• I would think a 2.5mm coil ID would be a difficult and careful operation (with extra caution re: DT in-hang). . . .. yielding a spacious 0.75mm above and below the coil.

• I would think a 2mm ID would be a still-be-cautious-but-doable size, w/ 1mm gap above and below the coil.
This is all just supposition and napkin-drawing. I'm not a practiced center-mounted-topcoil builder. (I do, however, seem to have a lot of hyphens to toss around today)