Create a uicontrol('Style', 'text') with FontName and FontSize as appropriate. The Position of the control does not matter, and you can create it with visibility turned off. Make sure the Units property is set to pixels.
Now, set the control's String property to the text you want. Then, get() the Extent property of the control. The returned height and width are the height and width that would be required to render just that String by itself. This will be the case even if the Position of the control was too small (or too wide) for the string: Extent will tell you the size you actually need for it.
You can of course re-use the same uicontrol to size different strings, by setting the String property and reading out the Extent afterwards.
Delete the unneeded uicontrol after you have sized everything.
