I think it is in res_mods\configs\DamageLog\message.cfg.
Reduce the font size from 40 and 30 on line 39..
"format" : {
"text" : "<p align='center'><font size='40' color='{costShellC}'><b>{damageHit} {splashHit}{criticalHit} {typeShell}</b></font><br><font size='30' color='{teamAttackerC}'>{reasonHit}</font></p>" // текст всплывающего сообщения.