If you want to edit your unit number using a multiplier then you need to:

1) using the latest ESF editor open up your save game.
2) Click on compress_data >> campaign_env >> campaign_setup >> campaign_setup_options
3) Change the second number to whatever you like. since the max unit size number is 200, the highest possible number you can use is 1,25

unlike Rome II and Attila, these choices will immediately apply to every unit in the game, not just the new units, leaving the already recruited units in their previous size