Sizing is really easy. If you have a sump that has plenty of water when it is operating and that can hold all of the overflow when the pumps are shut off, you have an appropriately sized sump. In most cases that means a sump that is a significant fraction of the main tank volume. Maybe you are looking at 1/4 of the volume for the sump. When the return pump is running, you need a big enough sump to keep the return portion of the sump full enough for your pump. When the return pump loses power, you want the open space in that sump to be big enough that the main tank stops draining before the sump runs out of room. That way things recover when the power is restored to the sump's pump. Do not skimp on sump size unless you are certain that the sump level will be closely monitored and adjusted. The place that water level varies in a sump system is in the sump, not the main tank. Unless you are prepared to constantly maintain the sump level, it must have room enough for normal evaporation losses in the main tank. That can easily double the minimum size of your sump if you are trying to go at an absolute minimal size.
There are two levels that must be determined experimentally. The first is the level that results in a totally full sump on a power failure. That is your sump's maximum normal level. The other is the level at which the sump barely has enough water in it for the pump and filtration to work properly. Sump level must be maintained, at all times, between these levels. If the levels are too close to each other, you are setting yourself up for major level maintenance tasks. If the room between those levels is huge, you have a larger sump than you really need.