Printed billboard or the newer digital billboards?
If for print, you need to talk with the print provider as regards file submission specifications (but it will not be 1:1 @ 300 DPI).
If for digital, you need to talk with the provider of the service as regards file submission specifications.

In general, a printed billboard will need a final scaled-up print at 36-45 dpi. Which means that at whatever scale you use, any bitmaps need in-application effective resolution scale*dpi. A simplistic example is if the job needs to be 400" (p[dfs can only be 200" in any dimension) and the design scale is 50%, a bitmap needs to be twice the target resolution in the design application so when scaled up it is the target resolution.

In your 48'x14' case, I would design (without the scale option) with a 12' x 3.5' (144" x 42") dimension page--25% of finished size. Which can be done in newer Xara applications. The size seems odd, at least I've not done one that is so long versus height.

Here's a quick run through my spread sheet. The bitmap section gives pixel sizes needed for areas as an example for included image use.

Click image for larger version.  Name: Capture_000434.png  Views: 197  Size: 43.5 KB  ID: 126254

I may be offline the remainder of tonight but will check in first thing tomorrow morning.