John Cutler
As stated above, the EMGS Back-to-Back is 16.5mm.
I have read somewhere that you might need to reduce that to 16.4mm if you use RTR wheels which tend to be to the RP25 standard, but I have very little experience of that.
Some people use a BB of 16.7mm but I would not recommend that if you are not going to build your own track.
I confess to using 16.5mm as a minimum so my BB often wanders towards 16.6 or even 16.7mm.
I do not think it gives me any issues that I am aware of.
No doubt that will horrify some people but hey, this is not P4!
As to frame width, there is no standard.
I find the 13.5mm width often supplied in kits a waste of space resource.
All too often one ends up using spacing washers to reduce sideplay on drivers; why not just make the frames wider?
If you build (or in my case, try to build) split frame chassis, you need all the frame width you can get so as to fit a gearbox and a split on the driven axle.
Likewise if you want to build working inside valve gear (I have no experience of that; you must be joking!).
The sensible approach is to work out how much sideplay you need on the driven axles.
That will be determined by the minimum curves you hope to traverse and the loco wheelbase.
I have built a Connoisseur LSWR G6 0-6-0T with 15.3mm wide frames (i.e. the kit default P4 frames) that traverses 30″ radius curves in EM with no problems (much to my surprise I admit!).
Good Luck!