If you’re new to soldering, a good “tip” is to use a copper coil rather than a sponge to clean the tip. And keep the tip tinned (keep adding a bit of solder and jabbing it off in the coil.) this prevents thermal shock and prolongs the life of the iron’s tip!
If you have a tip that is too small, then you will not have enough heat, i.e. thermal flow, getting to your solder joint. Note that heat is different from temperature! A bic lighter is hotter than your oven but it will not roast a turkey.
Symptoms of too small a tip - it will take too long to heat the solder joint. Pads and traces can lift and other things thermally connected to the joint may be damaged just like they will if the tip is too hot, but your flux may not activate and your solder will flow badly (or not at all) or make a cold joint like the tip is too cold.
Too large a tip and you can't do fine work or you rapidly heat everything around your part to soldering temperature.
