I have recently purchased a new frame and i have to source an appropriate wheelset, I am confident that i need a 20mm thru bolt on the front but i am slightly confusde when it comes to the dimensions on the rear, Could someone please explain the different sizes ( 140x 10, 150 x 12, etc)
Are there some standard sizes? Cheers

The big number is the width between the rear of the frame in mm
The little number is the dia of that axle or hold in drop out if you prefer.
Common ones
135 – 10 most common takes standard qr wheels
142 – 12 seems to be the new stadanrd on middleto high end
150 – 12 less common now mostly dh bikes

What frame is it?