Hello Colin,
I built the Explorer type boat, not the Sailing Row Cruser, so my experience is not right on the nose.
A1) Workshop size -
For the length - I recommend the length of the hull plus 2 meters
For the width - I recommend four times the width (actually four times the width of the saw horses on which the hull will be built)
Caution 1: this does not include material stoarage.
Caution 2: this does not include a space in which to twirl around while carrying a sheet of 8'x4' ply. If you get all your pieces CNC cut you wont have to twirl much and you'll need less space.
Caution 3: this does not include the space needed to construct and store the ama(x2).
A2) - As far as I know the DXF files produce parts with finger joints on the hull planks, butt joints on the deck panels.
FYI - I cut all my pieces by hand from 8'x4' sheets of 5.5mm ply and it was all managable.
FYI - all my joints are butt joints, I followed the build instructions, no issues at all.
Thats all I got for you Colin.
Happy retirement and happy building,
Tony