Well it depends on the type of plates you have, if they are bumper plates they will be the same size, when it comes to metal plates they could be the same size and thinner, or smaller diameter and thicker. Why don't you measure the diameters of both plates and halve the difference, that will be your deficit.
I think a better way of pulling from a deficit is to stand on some plates or rubber mats with the 45s on the bar.
As for how much weaker you are, I think for every 2 inches deficit you lose about 5-10% of the load you can pull, but of course it depends on the lifter and where their strengths are, the bigger the deficit the more your lower back will be used, at least that is what I have found. Others may have a different opinion.