Actual Size of Felt Pots and Risk of Soil Overflow

No the soil won't overflow from root growth. If anything, the roots will grow through the fabric and out the bottom of the pots, which is also not a big concern. You aren't going to grow a big monster plant in a 3 gal pot anyways. Roots will grow and fill every conceivable space within the soil and when there is no more room they either grow through the fabric and out the bottom, or they just become root bound with nowhere to go which will then severely slow growth. A good practice is to leave a inch or 1.5 inches of room between the top of the pot and the soil line. That will allow for you to water without flowing over the sides and making a coco perlite mess on the grow space floor. Keep your plants reasonably short. I grow in 3 gal fabric pots and i flip to flower when my plants reach 12 inches. When they finish stretching they are 30 inches tall, and when i harvest and empty my soil from the pot, there is no loose soil that falls away. The entire space is one giant root ball.