I've been using the free Yahoo mail since 1998, with a 6MB mailbox. And a month into using it, I was at 99% of my limit.
Deleting just enough old emails to free up a little space has been a constant chore that I do once a week. Since I can delete only a screenful (25, the default screen size) of my emails in one click, I would stop as soon as the percentage drops down to below 90% or the size drops to below 5MB, whichever caught my eyes first.
Starting last week or thereabouts, I've noticed that Yahoo Mail has increased my mailbox size to 100MB. And instead of running at 95% capacity, I'm running at 6%. Now a week later, I'm at 7%.
If past experience is any indication, my mailbox will grow 1% every couple of days until it finally reaches 100% in three to six months. At which time I'll be frantically deleting old emails, 25 at a time, till it drops to 99% or my wrist is sore, whichever happens sooner.
I'll be using the old Yahoo Mail the same old way. Except that now I will have 99MBs instead of 5MBs of old emails wasting Yahoo Mails disk space.