Is there an app that quickly makes an image of selected text?
You can use Automator.

Open Automator.
Choose Application from the templates.

Add a Ask for Text action. Tick 'Require an Answer'
Add a Create Banner Image from Text action.

Tick 'Use text input as image filename'.

Open the actions options options.

Tick 'Show this action when workflow runs'

Add a Lopp Action.

Control + click on the Loop Actions Title bar and select 'ignore input'

From the Loop Actions's drop down menus, select 'Ask to Continue' and Use the 'current results as input'

Save and run.

You will then be prompted to enter your text.
Then after you ok that you will be prompted to Choose you font.
Use the font size to determine the image size.

example 64 in Helvetica Bold gives me about 46x77
The same font but at size 300 gives me 177x360.

In each case I am only using 1 character.



If you want to do it from 'Selection'

Open Automator.
Choose Services from the templates.

Set the 'Sevices Receives input' to :rich text * and 
in* any application

Add a Create Banner Image from Text action.

Tick 'Use text input as image filename'.

Set the Text Style drop down menu to : 'Use Style of Rich Text input'.

Optionally you can add a 'Reveal in Finder' Action

The Size is set by what ever the original fonts are.



To use the Service.
  You can select some text, and control click to get a contextual menu. Listed is the services sub menu. Under this you should find your new service that will run on the selected text.