I agree some users what to see how big messages are including sorting to delete the biggest emails in their account to free up email space.

I also think it would be nice, wouldnt hurt to show message size. In my case, I want to know usually because of our filtering system which doesnt run for msgs over x KB in case of bayes. But there are other reasons, simply sometimes you want to know how big a message is before forwarding it, for example.

You could add it next to the date or below it in the email preview list. Its just a little something that imho would be nice, that other webmails have.

We don't like how many things are done in RC. Moreover, many clients select us for the same reason. Each new setting, each new option is an extra burden for the system. We can implement only those features which are needed not just for 1% or 5% of customers but, let's say, for 30% or more.

Anyway, if this feature gets many votes, we'll consider adding this setting. If not, please note you always have an option to request sponsored development.

Even if Gmail and Outlook Web Access miss to show message size it doesn't means it's not important for someone. Please check how is this solved in RoundCube. User can customize what's shown and even the order. It's a way better solution than to force what we think is important.