Our parrotlet is so so spoiled! She has 3 cages - one large flight cage for nighttime and/or when we're away for the day; 1 medium cage that she uses daily and we take around the house with us; 1 small cage for those trips to the vet (annual checkup).
The important bit is the space between the bars - it should be about 1/2", nothing larger or their heads can get stuck in between the bars.
The flight cage is 3' long x 18" wide x 2' high on a stand.
The daytime medium cage is 18" long x 12" wide x 3' high and sits on tables, counters and other surfaces with us.
The small cage is 12" long x 8" wide x 12" tall. I made a quilted cover for it so if I have to take her out in the cold, she's protected. A towel would work as well.
Hope this helps!