sorry, not following you…  do you have a link (well, probably wont help since guests cannot upload)… or screenshot?  no reason why it wouldnt look just like here…

are you running stock theme or a custom one?

I using 

css-only 1.0.15 theme

okay, false alarm. Updated theme to latest and it fixed now. But font is really really small.

font for whole site?  it inherits from the wp theme and adjusts as needed…

so two options…

1)  get the font resize plugin…  lets users adjust font size of site to their preference… remembers from visit to visit…

2)  create your own theme from ours and tweak the css size… to create  your own from ours, see:  http://codex.simple-press.com/codex/themes/theme-basics/creating-a-theme/

so you would create from the css only and then edit the stylesheet… if you want to go this route, do the creation part and come on back and we can tell you what to chagne…

Have added the font size plugin. But still think I will adjust the font as it looks to me like 8 or 9px on default.

I looked up the css file via the editor and found this.

#spMainContainer small {
font-size: 90%;
padding: 0;
}

What is this forum font-size settings as I like this size?

again, it inherits from the wp theme, so hard to give you exactly what you want… well, actually post content font size is 14.1333px

but just increase the 90% to some value that makes you happy…  try 95% or 100% and so on…

oh wait… wrong css… dont edit the small css…  find this:

#spMainContainer {
    color: #4c5d77;
    background: #f6f8f9;
    border: 1px solid #dde8ee;
    font-family: inherit;
    font-size: 100%;
    line-height: 1.2em;
    -webkit-gradientborder-radius: 5px; border-radius: 5px;
    padding: 1px;
}

and adjust the font size in there… that will generally affect the whole forum… the small part would be very little affect…