I have 2 email sub accounts. Both are set to 250 MB for a mailbox size. How can I increase those sizes? My main email mailbox is 2 GB.
You can't. Verizon sub accounts only get 250 MB of space.
You can set up an email client on one of your devices and download the mail off of the server, but then there will not be a copy on the server should you access the account from another device.