Hi all
I finished printing the 3D parts and started sourcing hw and parts.
I am still not sure of the work area size I should implement though.
One of the things I would like to do is neon inlays in hard wood. On such projects I currently 3d print templates and semi manually carve with a trim router and copy ring using the clamped templates (exemple in photo). I am currently working on a sign about a meter long and 30cm wide. Using the cnc, I guess I could carve it in several parts, but it would be more handy to do it in one go.
A good size for the work area of such project would be 800mmx400mm, z would be kept at the recommended minimum.
Is that reasonable for a first build? I would mostly work on wood, so slow speed would be ok for me if I work with aluminum.
Things like that, cuts are 5 or 6mm wide depending on the neon used and 7 to 10 mm deep (usually I do 3 to 4 passes to the final depth)
Pretty much a manual cnc 
800mm is wide for a Primo, you should definitely consider a LowRider. Mine was 750x550 and I had a lot of chatter in the middle, but it was still working.
I’d rather decrease the size than move to lowrider, I printed all the parts for MPCNC already
Were you using stainless steel tubes?
Also, I wonder if the rigidity is more or less a function of the sum of length+width, or longest dimension? For exemple, is a 800400 more rigid than your 750550 (1200 vs 1300 total), or worse cause 800 is pushing the limit.
The instructions page said to stay below 1m, so I though 20% less on the longest side was not too ambitious, but it is great to have your feedback on a real build 750 long
I was using stainless steel, yes. And as I said: it wasn‘t bad, there was just chatter in the middle. Could have been something else as well. There was never a bad drop of the cut depth in the middle, so just try. It‘s a great machine, I wanted to be able to place 120x60cm boards under my CNC because it‘s a common size being sold here, that‘s why I switched. 
You switched to LR?
Thanks for the feedback. If I reduce the size, it will not be by much.
If I am not happy with the performance, I can always cut some length for x tubes and belt and recalibrate. Easier than extending them.
Yes. At the moment I am reanimating my Primo. You can find both builds when you search for Schneewittchen and Der Froschkönig. 
For the outside rails, you can mitigate some of the size issues by using corner braces and midspan supports. Thingiverse has models for both. These do not help with the moving crosspieces. You can try at the larger size, and downsize if the machine struggles with your application.
gesundheit bahahaha! 