I am installing plumbing for a slab on grade new house. In order and basically in a straight line, farthest from the septic tank I am installing one tub, one toilet, one bathroom sink one double kitchen sink then to septic tank.

Where should vent be located and what size pipe?

Let me show you a typical, (in my area) bathroom group rough in. This configuration only uses one vent for the entire group.
4" ,(under cement in slab) 3" (on first and second floor) from the toilet to the main. The lavatory to connect to the toilet drain line and a 2"vent to the roof or revent back in the attic to a existing roof vent. The tub/shower to connect to the lavatory drain line and be wet vented by it as will the toilet. Be advised to always check local codes before changing or remodeling your drainage system.
The kitchen sink will have its own 2" vent that may be tied back to the lavatory vent before it's run out the roof. Good luck, Tom