Make sure that you allow websites to choose their fonts.
Tools > Options > Content : Fonts & Colors > Advanced > [X] "Allow pages to choose their own fonts, instead of my selections above"
It is better not to increase the minimum font size, but use an extension to set the default page zoom to prevent issues with overlapping text.
You can remove all stored data from a specific domain via "Forget About This Site" in the right-click context menu of an history entry (Show All History or History sidebar) or via the aboutermissions page.
Using "Forget About This Site" will remove everything like bookmarks, cookies, passwords, cache, history, and exceptions from that domain, so be cautious and if you have a password or other data from that domain that you do not want to lose then make a note of those passwords and bookmarks.
You can't recover from that "forget" unless you have a backup of the affected files.
It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.
Re: email account has a blank space taking one third of the page, I do not have enough space to read my email.
Thank you for answering but my problem has nothing to do
with the font size, but space, blank space, wasted with nothing on it
meanwhile I have no space to read my email.
The blank space is just above the email.