Well I find the Junk filter works quite well. It's a dynamic filter so you need to train it. It learns bit by bit. But yes, the size filter was my 1st request on the forum roughly 18 months ago. It would be handy for multiple reasons. But remember, if the intention is just to filter out virus, then they will increase the size of the virus.

The network wasn't as full of virus as it is now. There's been a concentrated effort to spread virus & spam by dubious sources over the past 12 months (by autogenerated spamming results.) Particularly the past 6 months.