To change the text size you need to use the size tag in the BB Code options. Select the text you want to make larger or smaller than the default size and push the Size button in the formatting bar on the new topic screen. The size button is the one with the A and a double-headed arrow alongside it, like this:
This will give you something like this:
[size=10pt]My text here[/size]
You can change text to bigger or smaller using this same code - just change the pt size number. 10pt is the default offered, so 8pt would make it smaller, 6pt even smaller still, 12pt would make it slightly larger, 14pt bigger again, and 18pt even larger, and so on... for readability 8pt would be as small as you'd want to go I think or we'd all be pressing our noses against the screen trying to read the text!