OK looks like it will fit fine, here is the complete 49 sensors on a 30CM wide pcb at minimum spacing:
as you see we have room at the end if needed. So to go forward I need to know what spacing between the sensors you would like and how the 8 connections to the mega are made. The choices would look to be a 8 pin header (which provides either holes for wires or a header to plug a cable in to, or an 8 pin dual row header to take an 8 conductor IDC ribbon connector cable. Which would you prefer? Also where do you want the connector, left end, right end, middle of the board or somewhere else. All the parts need to be placed before routing the pcb. Does anything else (such as the switches in the schematic) need to go on this board as well or is it only the hall sensors and the diodes.?
Peter