Master Cylinder sizing
 

Im doing a brake upgraded with a Wilwood 6 piston and 4 piston combo.

Front calipers pistons are 1.75/1.38/1.38
Rear calipers pistons are 1.38/1.38

I have 7/8" manual with C5/LS1 combo now.

Will the 7/8" work?

So, your effective caliper piston areas are ~5.38 and 3.00 square inches front and rear respectively (W6A/W4A's?), versus 4.00 and 2.46 square inches for the C5/LS1 combo. That's a 35% increase in area in the front and about a 22% increase in area in the rear, so if you want similar pedal travel and effort, then a 1", 1.03" or 1.06" bore MC might be a better choice.

That's not to say that your 7/8" bore MC won't work, just that your pedal travel may be a bit excessive, possibly to the point of being dangerous.

Tobin
KORE3