One of my blog recipients has a Gmail inbox that is over the limit in size. Either some of the contents should be deleted or more space should be rented from Google. As far as I know, Google Drive allows 15 gigs of space but more space can be rented on a continuing basis. I try to keep my stuff within the free limits. The free limits apply to Gmail's total (inbox, send, drafts, others), Google Drive and other Google services. Despite writing a blog post daily for about 95% of the days between April 2008 and now, I haven't had too much trouble with saving my blog. The thing that expands all the time is the Gmail.
I arbitrarily chose 1000 saved emails as the "dump!" signal. The difference between looking at Gmail on a computer and on a tablet such as an iPad is stark. Computers are much more powerful. If you haven't seen the difference, try signing into Gmail on a computer and take a look. The interface and its choices and buttons is easier to use on a computer. I like ASUS computers, including used ones. They are inexpensive and powerful enough for my use. I don't play video games and I don't use my computer in competitive games so I don't need enormous space or enormous speed.
When I am in Gmail on a computer, I can easily see how many messages I have in the Inbox. When that number reaches 1000, I delete it down to 500. The interface makes it easy to change the display from 'newest' messages first trailing off in age to "oldest". The display also makes it easy to select all the email messages on a single page. In the upper left corner of the Inbox section is a handy set of choices. If I just click in the box above the choices, one page of messages is selected. Remember these are the oldest ones I currently have. They usually are from about one month ago. Hitting the delete key on the keyboard takes them all out. (there is a choice to select the entire Inbox contents but I don't do that). I keep selecting one display page at a time until I get the contents down to 500. I try to remember to look at the other sections of Gmail, too. The send messages and even saved draft messages can pile up but not like the Inbox.