If you're looking for a meeting place or a good stop around ½ or halfway between Philadelphia and Portland, ME, you can try Hartford, CT, which is about 3 hours northeast of Philadelphia, or about 3½ hours southwest of Portland, ME.
Another possible place to stop near the midpoint is New Britain.
About ⅓ or a third of the way, you can stop at Yonkers, NY. This is about 2 hours northeast of Philadelphia.
About ¾ or three fourths of the way, you can stop at Lowell, MA. This is almost 5 hours northeast of Philadelphia, or almost 2 hours southwest of Portland, ME.
If you drive straight through without stopping at all, the total distance is 408 miles. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 6 hours, 30 minutes.