This feature is already there in Synaptic. You can add column with name "Download Size". More over the synaptic also informs how much it will download before actual downloading!
yes, the synaptic thing is nice, (I use it) but it DOESN'T show the download size of the dependencies... which is annoying when you think your downloading 10MB, and your now donwloading 3 times that much....
I was going to ask for the same. Let's make this as a file viewer: you can have optional columns and sort them by size, popularity, type of application (supported by ubuntu, unsupported, third party), and others...