Plain TeX (in accordance with its description) doesn't do anything fancy with fonts: it sets up the fonts that Knuth found he needed when writing the package, and leaves you to do the rest.
To use something other than Knuth's default, you can use Knuth's
\font\foo=nonstdfont ... \foo Text set using nonstdfont ...
The name you use (
nonstdfont, above) is the name of the
tfm file for the font you want.
If you want to use an italic version of
\foo, you need to use
\font\fooi=nonstdfont-italic ... \fooi Text set using nonstdfont italic...
This is all very elementary stuff, and serves for simple use of fonts. However, there are wrinkles, the most important of which is the matter of font encodings. One almost never sees new fonts that use Knuth's eccentric font encodings - but those encodings are built into Plain TeX, so that some macros of Plain TeX need to be changed to use the fonts. LaTeX gets around all these problems by using a “font selection scheme” - this 'NFSS' ('N' for 'new', as opposed to what LaTeX 2.09 had) carries around with it separate information about the fonts you use, so the changes to encoding-specific commands happen automagically.
If you only want to use the EC fonts, you
can in principle use the ec-plain bundle, which gives you a version
of Plain TeX which you can run in the same way that you run
Plain TeX using the original CM fonts, by invoking
tex. (Ec-plain also extends the EC fonts,
for reasons which aren't immediately clear, but which might cause
problems if you're hoping to use Type 1 versions of the fonts.)
The font_selection package provides a sort of halfway house: it provides font face and size, but not family selection. This gives you considerable freedom, but leaves you stuck with the original CM fonts. It's a compact solution, within its restrictions.
Other Plain TeX approaches to the problem (packages plnfss, fontch and ofs) break out of the Plain TeX model, towards the sort of font selection provided by ConTeXt and LaTeX - font selection that allows you to change family, as well as size and face. The remaining packages all make provision for using encodings other than Knuth's OT1.
Plnfss has a rather basic set of font family details;
however, it is capable of using font description (
created for LaTeX. (This is useful, since most modern mechanisms
for integrating outline fonts with TeX generate
in their process.)
Fontch has special provision for T1 and TS1 encodings, which you select by arcane commands, such as:
Ofs seems to be the most thoroughly thought-through of the alternatives, and can select more than one encoding: as well as T1 it covers the encoding IL2, which is favoured in the Czech Republic and Slovakia. Ofs also covers mathematical fonts, allowing you the dubious pleasure of using fonts such as the pxfonts` and `txfonts.
The pdcmac Plain TeX macro package aims to be a complete document preparation environment, like Eplain. One of its components is a font selection scheme, pdcfsel, which is rather simple but adequately powerful for many uses. The package doesn't preload fonts: the user is required to declare the fonts the document is going to use, and the package provides commands to select fonts as they're needed. The distribution includes a configuration to use Adobe 'standard' fonts for typesetting text. (Eplain itself seems not to offer a font selection scheme.)
The font-change collection takes a rather different approach - it supplies what are (in effect) a series of templates that may be included in a document to change font usage. The package's documentation shows the effect rather well.
Simply to change font size in a document (i.e., not changing
the default font itself), may be done using the rather straightforward
varisize, which offers font sizes ranging from 7 points to
20 points (nominal sizes, all). Font size commands are generated when
any of the package files is loaded, so the
11pt.tex defines a
\elevenpoint; each of the files ensures there's a “way back”,
by defining a