David;
I'm surprised that your
client "does not know much about the project" ??
The original drawing looks like it was 'architecturally prepared' (not some interior designer's artsy-fartsy sketch) so it surely is possible to ask your client to provide two REFERENCE dimensions. Overall dimensions would give you the least error.
Some scanners and most faxes give errors (usually about 2-3% over) in the 'length' direction so it is important to get TWO reference dimensions. See attached image. One foot = 304.8mm.
Once you have a 'scalable' image it is quite easy to resize (in both X and Y directions) in Archicad. Note that you probably will want a fairly high resolution image if you want to reproduce the drawing accurately.
HTH - Stuart