I have a brake master cylinder off a fiesta, it has 4 ports, 2 are m10 and the other 2 are smaller, i have connected the front brakes pipes up to the fron ports and use 1 of the back ports for the rear and blanked the other up.

If i have use 1 small port on the front and a larger port on the from will it brake ok?

Yes, it'll be fine.

See my post
My master cyl

Is this one the same as yours?? the ports are different sizes but they are 2 x 12mm and 2 x 10mm

Why not fit the brake switch in the spare one.