This test execCommand('FontSize', ...).  You should see 'small' 'medium' and 'large' below in every increasing font sizes.

initial:
|

fontsize 2:
|

type small:
| <font>
|   size="2"
|   "small<#selection-caret>"

fontsize 4:
| <font>
|   size="2"
|   "small<#selection-caret>"

type medium:
| <font>
|   size="2"
|   "small"
| <font>
|   size="4"
|   "medium<#selection-caret>"

fontsize 6:
| <font>
|   size="2"
|   "small"
| <font>
|   size="4"
|   "medium<#selection-caret>"

type large:
| <font>
|   size="2"
|   "small"
| <font>
|   size="4"
|   "medium"
| <font>
|   size="6"
|   "large<#selection-caret>"

console:
|