If you use the likes of MPC for playback, under 'View' > 'Options' >'Subtitles' > 'Default style' you can change the size from the default (100) to another lower number. If you are referring to changing the coding of the .srt file, then it has to be done on a per line basis using Notepad (I don't know of a program that will automate it). The problem with this, apart from the labour, is that some software players and hardware players may ignore it and use their built-in settings.
