How willl a Mac Mini's SSD effect the maximum process size when use swap? Reading, possibly out of date info, elsewhere OS X seems to limit the maximum process size according to the total RAM and a fixed proportion of the available disk. 

The new 2018 Mac Mini is very tempting, but what will the effect of limiting myself to a 128G or 256G in terms of process size?

I'm currently use Wolfram Mathematica to do so some weighty audio simulation on a 2009 16Gigabyte 2.88Ghz 4 Core Mac Pro with OS X El Capitan installed on a 256G SSD and piles of extra SATA HD. Every so often Wolfram come to a dead stop and quits, which may be a memory issue but I'm not sure.
macOS does not limit the maximum process size (i.e. working set) according to total RAM and a fixed proportion of available disk. This is 100% not the case in any way at all.

You can set arbitrary limits on process size using the "ulimit" command in the Terminal, those are however limits chosen by the user - and thus can be changed as need be.

There are ofcourse actual, "physical" limits of a process size given by the hardware. Contrary to common belief, it is not actually possible for an ordinary 64-bit system to have processes that address 2^64-1 bytes of RAM. On macOS (and Windows other systems) processes are actually limited to 2^48-1 bytes instead. I.e. a given pointer only has 48 bits that are actually significant.

There are paging techniques to counter this, but in practice no ordinary program employs them.

In addition to the physical limits, there are other limits you'll run into. If you have meaningful data in RAM, the system needs somewhere to store it. The total RAM installed on the system as well as the free disk space available for swapping becomes a limit.

Note that this is not a fixed limit in any way. If your RAM contents are compressable (i.e. for example if it contains many repeated bytes), then you'll be able to have extremely large process sizes without problems. Much more than you have physical RAM and swap space.