A picture, being just a bunch of dots, has no scale like a vector plan might, so predicting the way Photoshop might see it is difficult. Scaling in Photoshop is easier with the CS3 version since there is a measuring and scaling tool - but you still need to assert the actual size of the drawing image.
Archicad's export to JPEG or any other pixellated format depends on several factors:
-- the extent of the plan view field
-- the size and resoluton of your display.
-- the zoom factor.
Another problem is that you often need to capture many pixels at higher than screen resolution.
To cope with this challenge, you'll want to put a large graphic scale on the floor plan before you export it and scale to that. This might let you anticipate how your setup exports an image.
Another way is to ignore the scale in Photoshop. Re-import the edited image to Archicad and make it match your other drawings there by eye.
Many users export PDF instead.
Dwight Atkinson