There are two main moulds for MP Optimus Prime, the original MP1 mould and the newer MP10 mould.

Left: MP10, Right: MP1

In terms of size...
MP1 = 30.5cm
MP10 = 24cm
CW Megatron = 23cm

So yeah, height-wise MP10 is the nearest match. Also, remember that Australia will be getting the reissue Japanese MP10 Optimus Prime which has different colours from the standard Hasbro release. You may like to compare and see which version you prefer.