Size

The "Size" (サイズ Saizu) is a numeric value used on monster cards.

Size over
When the total size of monsters on your field is greater than 3, you must put each monster, except the last monster called, from your field to the drop zone, until the total size becomes 3. This is done as soon a monster enters the field, before any ability can be used.

Certain continuous effects, such as Solomon's Great Barrier, allow to alter the size of your monsters. However, when that card leaves the field or its effect is nullified, you must check for the total size is at least 3 immediately.

A size 4 or greater monster cannot be called to the field without prevent it from being sent to the drop zone, unless a card effect allows it.