The 172s I fly have a MTOW of 2400LBs. With full fuel this roughly translates to two 15 stone blokes in the front, and two (petite) female passengers in the back
. Or else one larger passenger in the rear, with no baggage.
The 160bhp PA28 I imagine would be similar.
EDIT - and having just played with the W&B calculator I reckon you could get someone up to around 17 stone into the front, allowing for a 15 stone instructor, without moving the CofG too far forward. So quite a bit of scope for 'larger' students, with a less than trim instructor!