25.3 UNIT SIZE
These entries list the minimum number of models a unit must have. When you select a unit to be pat of your army, you must take it at the minimum unit size unless it can be taken as an understrength unit or a reinforced unit (see below).
Since minimum unit size is defined as the minimum number of models a unit must have, we should be able to determine that maximum unit size would be fully reinforced. So for a unit of horrors maximum unit size would be 30 models. So even if you dont accept that Split and Split Again will allow you to add models above maximum with other abilities, you can still add models until you have 30 models.