Some older receivers didn't allow you to set a particular xover frequency and it was fixed at 90 Hz. If yours does allow you to choose a xover frequency the setting should be in the same menu where you set speakers to Large or Small and indicate which speakers are present. On mine, it's a menu called 'speaker config'.
Large vs Small doesn't really refer to the size of the speaker but rather 'send the bass to this speaker or not'. Any speaker set to Small will have any frequencies below the xover sent to the subwoofer. If you don't have a subwoofer they get sent to the front speakers (and in that case most receivers will automatically set the fronts to Large). If you can set speakers to Large vs Small but there is no setting to choose a xover frequency then your receiver is the type that uses a fixed xover. It will still work the same as other receivers that allow you to choose your own xover, you just get its fixed value instead of being able to choose your own.
80 Hz is almost always a good choice so if the receiver is fixed at 90 Hz that isn't too bad anyway.
