if you want to maintain without size you are almost constantly in a cutting stage or there abouts. Keeping the intake of protein high but including more cardio like based workouts to drop the bf%. This will keep you strong without increasing mass! super setting in a cutting phase is great for this with light weight 15 reps or so no rest. example below
a1 - 15 reps
a2 - 15 reps
90sec rest repeat 3 times
a super setting day in the gym will usually take you about 40mins or so in the gym 4 - 5 times a week depending