Might just be a question of wording, but I keep looking for the feature to set task weights.
I think I misunderstood and by "size" you just meant "point value". This will happen eventually as part of other projects work (T9378, T3670) but those need to happen first. I'm not yet sure if we're going to make this a first-class value or let you designate a custom field as a "points field" or something else.
I'm just going to close this in favor of those other broader tasks.