Orbital size of Carbon and Oxygen in Methanal
The orbital size in the picture shows the coefficient in the linear combination of atomic orbitals, i.e the pi bonding MO has a larger contribution from the oxygen atomic orbital, and the pi anti-bonding MO has a larger contribution from the carbon atomic orbital. 

Here is another picture for a different molecule where they use the same convention, but also show the coefficients explicitly (source):




  Is the diagram for π bonding drawn correctly in terms of orbital size?


An orbital is not a "thing" so it does not have a size. It is a 3-dimensional function. The picture just shows the symmetry of that function and the node (where it is zero). For a slightly more realistic depiction, you can check out molcalc.