Nobody should really by using i386 for new systems. The advantages of running amd64-compatible hardware are too big to ignore. Lower power consumption, much faster in most cases (the extra register used by PIE hurts i386 mode a lot more than amd64 mode with its extra registers), more address space for ASLR, some other security mitigations don't work on i386, more compatible with software in ports.
If it's an old system then you already have it so you can just see for yourself how much space it needs (and make your own decisions on swap space vs RAM, and whether to disable relinking at boot [just using it for upgrades/syspatch], and whether to bodge things to use ld.bfd to save RAM). So much depends on how you're going to use the system that I don't think it's really useful to publish the numbers.
