PR size for 4 inch woofer.

Hello,
I took apart my old philips two way bass reflex speakers that has 20 watts 6 ohms 4 inch woofer. These woofers dont have a crazy amount of excursion but they sounded really very solid and pleasant in their stock enclosure. I am going to make a portable boombox and I will be adding passive radiators to keep things small. The only thing I can't finalise is PR'S size.
Some say to go for 4 inches and some say to go for 6.5 inches. I think going for a bigger PR would be better as these woofers were in quiet a big box for their size but still need some clarity on what would be a better decision or general rule of thumb.

Thank you.