Tiles are too large to transfer to GPS 72

I have a Garmin GPS 72 and I use mapsource to transfer tracks and waypoints to it via a serial port. I'd like to be able to transfer a map but the internal memory of the GPS is only 1mb. I use my GPS for cycling near home so most of the tile I want to transfer I don't need. So is there any software that could reduce the area of the tile hence reducing it's size and still allow me to copy it using mapsource (gmapsupp.img), as it's a serial point connection I don't know how else I'm going to get it onto the device. Thanks.