LyX (2.1.4) has several poster templates. For instance:
New from the template ...
I found poster-beamerposter.lyx useful since I could easily override the sizes in:
Document> Settings ...
width=121.92 and height=91.44 are cm units for 48 "and 36", respectively, with the custom option, since you must specify cm values in this option.
The template is quite confusing in LyX, as it contains various strategies. This requires some experimentation, but the end result is pretty nice. I found that using the i6dv style was useful, but you have to find / load / debug it yourself.
Proper placement of frames, columns, blocks, lists, etc. It is very important that the result works correctly in this template. Unfortunately, my copy of LyX does not automatically retain the correct nesting when trying to add a new Block inside a Column or Enumerated inside a Block . It's very easy to get things down to the level of external nesting and even break LaTeX.
Another thing that took me some time was to understand that each Block must have a Separator before the next Block , otherwise LyX does not know how to start a new block and will combine everything together.
I would venture to say because of these quirks that posters in LyX are hardly less complex than LaTeX itself. Perhaps there are some settings in LyX that I skipped to make this easier.