If I want to know the measure of a room in a blueprint, how do I measure. For example, the blueprint says 10'4".

The width of that room measurement is from stud to stud or finished drywall to finished drywall.

Thanks :)

Blueprints should have a scale. You use this scale to figure your measurements.

If the scale on the blue print shows 1/4" it should show what that 1/4" measurement is in actual measurement.

This scale should be on the blueprint.

Add: I reread your post. The blue print should show the measurement of your 10 ft 4inch wall and what that pertains to in that measurement. Is it the floor plan portion of the blue print? Usually a floor plan will show the completed house. So, it would be the width of a finished wall, if I remember correctly. What blueprint do you have?

In my area when we establish walls on a rough in we measure from the unfinished wall, (the plate) so the answer would be, "from stud to stud". Good luck, Tom