Not really following you I am afraid. The font resize plugin simply adds a user level to make bigger/smaller as one would do with the browser ( /-) and stores it in a cookie. It is not global.
Your base font size is inherited from the WP theme. If you want to change that tell me what SP theme you are using and I can advise on what you need to do.

I also need to change the base font size is very small. How should I proceed? I am using SP theme – Default 1.1.1 by Andy Staines & Steve Klasen. See the link for the forum – http://www.doceshop.com.br/blog/forum/

the font size is inherited from your wp theme…  and then scaled…  its not fixed since most users want the forum to look like their wp site…

first, and this is important, make your own theme instead of editing ours so you dont lose changes on upgrade:  http://codex.simple-press.com/codex/themes/theme-basics/creating-a-theme/

then edit the main theme style file (ie default.php) and edit the base font size…  if the theme supports overlays, like most of ours, you would instead edit the overlay file you are using…  ie default/styles/overlays/ice.php…  and find the base font size at:

$mainFontSize            = ‘100%’;

you can increase it to whatever you want…

Wow! Incredibly easy. Thank you, it worked. 

You’re welcome.

Thank you for the tip this was very helpful.

You’re welcome

Yeah… This was a huge help… had some complaints about the size, this fix worked like a charm!

Good news – glad it helped. thanks for the update…

Good news – glad it helped. thanks for the update…