Some groups have a resizing defined in the editor. This means that the group will resize to a certain number of planes at the beginning of the defined month, provided the unit is in a base with at least a level 1 airfield with sufficient supplies. If a unit has such a pre-defined resize, it will be shown in the top left of the unit information screen. By clicking on the text you can turn off the automatic resizing, the unit will then retain its current size.
Carrier unit resizingEdit
Group resizing for units on carriers is quite a bit more complicated. Carriers have a limited number of planes they can support. While the standard squadrons make good use of this space, a player may wish to select different squadrons and then it becomes hard to make optimal use of the space. To help players with this, it is possible to resize air groups on ships.
The first option is to let the game take care of how big the units should become. This is linked to the historical aircraft loadout of carriers, so there are a lot of factors taken into account, such as ship type, nation, year and plane type. The resulting size is then a percentage of total capacity. The rules are rather complicated and I'm looking for a way to represent it as easy as possible, in the meantime, check them here. If you want this to happen, make sure "resize to fit" ship is showing in the top left.
Another option is to simply choose the size of the unit yourself. To do this, make sure "resize to fit ship" is showing, then turn it off and on again. You should now see a text box where you can choose how big the air unit will become. Resizing should occur the next turn. But it will also occur on the current turn if the carrier is disbanded in developed port (probably with supply requirements). Tokyo, Yokohama, Hiroshima work nicely for instant resizing.
Some players like to resize many of the originally land-based (or ship-based in case of floatplanes) IJN airgroups to a maximum size of the carrier. Akagi is best for fighters/bombers resizing with size 81, Chitose/Chiyoda is best for float planes with size 24. This will create large airgroups suitable for training and/or combat. Always check with your PBEM opponent if he is OK with this, some players see this as gamey.