Electric Notes
1 Inch.
Strapped within three feet of each box.
Strapped within three feet of each coupling.
Strapped every ten feet.
Do NOT forget your box offsets.
You may be able to get away with 3/4", but it will be a snug fit
Remember your AHJ has the final say-so.
Don't forget that you will also need to run a similar size ground wire as well.
For four #6 wires you will need to use 3/4" EMT conduit, minimum.
Also, check your local building & electrical codes for the limit on the number of bends you can have in a single run.
This is done to prevent damaging the wires as they are being pulled.