There are quite a few factors to consider if you want to be absolutely sure about the RAM in your machine. You mention size difference and how much per chip, but don't forget the frequency the RAM operates at as well as the voltage it's set at and the timings e.g. 8-8-8-24.
I freely admit I'm not the world's foremost expert on this and I'm sure other people have their own opinions, but I have always been of the opinion when it comes to RAM timings, make sure they're lower and evenly distributed, e.g. 8-8-8-24 is better than 11-7-9-24.
And I like your build; it's very similar to mine