[fat city]

They could do different font sizes (within the limits of the ball size-- which was uniform for all fonts. However on rethinking that old system, I believe the Composer could do proprtional word spacing (for clean margins) but that would be for formal printing only due to the complex coding (at least compared to typing) and no one would use it for "memos" or private notes. We used it for important documents going into print.

[Buckhead]

From this web page, http://www.nationmaster.com/encyclopedia/Typewriter we find the following history of the electric typewriter:
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Selectric mechanisms were widely incorporated into computer terminals in the 1970s, ... .

Later models of Selectrics ... introduced selectable "pitch" so that the typewriter could be switched among pica ("10 pitch"), elite ("12 pitch"), and sometimes agate ("15 pitch"), even in one document. Even so, all Selectrics were monospaced -- each and every character was the same width. Although IBM had produced a successful typebar-based machine, the IBM Executive, with proportional spacing, no proportionally-spaced Selectric office typewriter was ever introduced. There was, however, a much more expensive proportionally-spaced machine called the Selectric Composer which was considered a typesetting machine rather than a typewriter.