User blog comment:LilyheartsLightning/Citadel and Reckoning DLC announced/@comment-1440333-20130225225803/@comment-1440333-20130228194140

I dunno if those are PC file sizes but on XBox:

Resurgence - 166MB

Rebellion - 159MB

Earth - 192MB

Retaliation - 437MB

Reckoning - 157MB

So Reckoning is the smallest of the lot!