you might not want to use the default character as its made a lot taller then an average human so if for example you made say a normal house door the default player would look too big for it or if you made an assault rifle it would look too small compared to the player(even more so if you want a female character) because in UE4 1 unreal unit is supposed to be 1CM so you might be better off if you just model to scale using real world dimensions rather then using the default character as a reference to try and get the right size for your models.