I can't see how they will fit all the things from just a 3.1 level kickstart without using tricks that the 4000T used, like moving Workbench.library to disk. That compiled C code will be bigger in almost every case.
Most of AmigaOS 3.1 is written in C. dos.library was written in BCPL up to 1.3 & converted to C for 2.0 (or even 1.4 beta?)
IIRC they used several compilers, one being lattice. Which turned into SAS/C. Back in the gcc 2.95 days I did some benchmarks and the software I tried was quicker with gcc than SAS/C.
There are many factors that will influence how large/fast AROS68K runs. It's far too early to even contemplate what it will be like.
However CD32/A1200's can have 1mb kickstart, A500/A2000's needs wires added. No idea about A3000/A4000...