To see if Vim is reading ANY script, look at the output of the :scriptnames command.
Did you actually edit your .vimrc file in order to change font size? What was the "set guifont" line before and after your change? What does :echo getfontname() tell you before and after your change? Font is a little more complicated than other options, you might just be entering a bad string.
I've never seen getfontname() return nothing.
Without arguments, getfontname() is supposed to return whatever font is actually being used by gvim, whether it was found in the guifont option, or is just the default font.
However it doesn't work if the GUI is not running. Did you run the ":echo getfontname()" command manually from gvim, after it is all the way started up, and not in your .vimrc, etc.?
Note, the best way to set a font, is to use the dialog, with ":set guifont=*"
Then you can make note of the exact font string used, by doing ":set guifont?" to echo it, and use that string in your .vimrc or .gvimrc.
Font strings differ greatly depending on the system Vim is running on.
I don't know what 'sessions' is, but I assume it automatically saves and loads a Vim "session" file via :mksession automatically when exiting and entering Vim.
Try removing "options" from your 'sessionoptions' option in your .vimrc. It is there by default and a major annoyance to me. You might tweak 'sessionoptions' to make sure other things you don't want restored blindly are also not included.
Another note, and I almost suggested this, now I wish I had: if an option is set differently than you expect, you can do ":verbose set optionname?" to see not only what it is set to, but also what file set it to that value. This would have found the culprit more quickly I think.
Yes, I figured that. But Vim has an option called 'sessionoptions' which controls what actually gets saved in a session file. You can customize the value of this option in your .vimrc, if desired. See :help 'sessionoptions'.
However, I'm coming at this from the viewpoint of having a detailed .vimrc and a lot of plugins all setting options and mappings. I don't need to have Vim remember options in a session, because the options I like are already set in my .vimrc.
You may like having options remembered, because then you can simply set the option manually, and the auto-saved session file will remember it and restore it the next time you run Vim.
Still, you might want to at least look at :help 'sessionoptions' to see what is available for you to customize.