It is not that clear in the documentation just which characters are needed but the following example cuts the subject line to 50 characters:
git log --oneline --format="%h %<(50,trunc)%s"
The format specification is %< and the arguments for that need to be in parentheses. In this case, 50 chars and truncate the excess.
For instance, performing this on the msysGit repository yields:
C:\src\msysgit>git log -n 5 --format="%h [%<(12,trunc)%aN] [%<(12,trunc)%cN] %<(50,trunc)%s"
218ed04 [Sebastian ..] [Sebastian ..] Merge pull request #154 from csware/tortoisegitp..
8a920b9 [Sven Stric..] [Sven Stric..] Installer: Detect TortoiseGitPlink from Tortoise..
448e125 [dscho ] [dscho ] Merge pull request #152 from csware/syscommand
db8d1bf [Sven Stric..] [Sven Stric..] Perl readline creates empty sys$command files if..
753d3d6 [Johannes S..] [Johannes S..] Git for Windows 1.8.5.2-preview20131230