If your subtitle name is "name.srt", just save the following in the same directory with the name "name.srt.style" :
Code:
ScriptType: v4.00+
PlayResX: 384
PlayResY: 288
[V4+ Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
Style: Default,Arial,10,&H00ffffff,&H0000ffff,&H00000000,&H80000000,-1,0,0,0,100,100,0,0.00,1,2,3,2,20,20,20,1
You can change the size by changing the 'fontsize" (3rd parameter) or the ScaleX and ScaleY (two first "100" on the line).