Question regards “Width (X axis or “Beam”) should always be the shorter axis.” - what are the caveats this is targeted at?
My plan would to start with 48" capacity on the X axis, but only say 30" on the Y with the thought that it’s simple to then move to full sheet capacity at a later date without having to change much.
Bad plan?
It CAN be done, but will make bootstrapping the build by cutting your own strut plates much more difficult, because the X axis is definitionally not capable of being long enough for the strut plates.
In terms of machine function, the shorter the beam span, the more rigid the machine is. Therefore for any given set of dimensions, the shorter the beam span, the more rigid the machine, regardless of the length of the Y axis. These are the primary reasons for this.
For the first, if you can “tile” the cut for the strut plates, or if you have access to a larger machine to do the cutting, it’s not a problem.
For the second, if you are keeping to a reasonable span (say less than 50") you will still have good to excellent rigidity, and so this probably will not cause you problems. If you want to have multiple tables, say one 48×96 and one 48×32, this is a reasonable build.
My build is 25"×49", and so I chose the 25" span as X, since this resulted in the most capable machine, even though I had material for a 49" span machine. I may build the 49"span beam later, as full sheet capability appeals to me, but it will be a separate machine.
Ah got it, that makes sense, thanks. Appreciate the explanation.
No, not a bad plan if you need the space for now. But as @SupraGuy mentioned:
(I’m guessing he meant rigid…)
and
If you’re going to be expanding to full sheet capability later, then perhaps you could temporarily use the longer Y rail and belts for the planned expansion to cut the strut on a larger table (or sheets of plywood on the garage floor), then revert to the shorter Y until you are ready to expand permanently.
If you build the X axis to the same dimension as the final build, you will have the same stability as you will once the final build is complete and the Y axis is longer than X. If you are planning to use the machine in the smaller build dimensions for a longer time, or are planning to cut some material that requires strong, stable, rigid cuts. then perhaps consider building it with a smaller X beam until you are ready to expand, then build the larger X beam at that time.
Lol. I’m terrible with the phone keyboard. Touch screen keyboards with no tactile feedback mean loads of typos. Original post edited.
I do it exactly like that, have a look at the Ghostrider (even though Ryan does not recommend it). With my LR3 I did teeny tiny inlays in earrings without any accuracy problems and went 3000mm/min in large pieces. So yeah, it works (though, from the maths side, it is not as rigid). 
Perfect! That’s exactly what I’m looking to do, and the glow in the dark filament is super fun, love it.
Thanks guys 
And actually - you’ve basically landed in a similar place as me. I am thinking a bit more Y than 30", but I’m thinking I’ll do a 49" X beam so that I always have the option to throw a couple of sheets on the ground, add a Y rail, some belt blocks, and do a larger project, without devoting the entire area of the machine permanently.