You can set font sizes in css. These css settings can be applied with the hi[ghlight] command. E. g.:
:hi CmdLine -append font-size:16px To view a list of all settings that are possible to change with the highlight command, see :help :hi.
For example, you would most likely also like to increase the size of the descriptions of the completion items too. This can be done with the following setting:
:hi CompDesc -append font-size:16px To use these automatically on startup, just open the file ~/.vimperatorrc and add them there, without the leading colon.