The largest extant spider (the Goliath birdeater) is well over 300 times the size of the smallest (contested, but take the Patu digua as a reference) as an adul

The largest extant spider (the Goliath birdeater) is well over 300 times the size of the smallest (contested, but take the Patu digua as a reference) as an adult. With the bee hummingbird at about 2 and a quarter inches (~57mm), that would mean the biggest bird would need to be over 60 feet (18.2m) beak-to-tail by your reckoning. And we know of pre-spider critters (more like modern camel spiders than modern true spiders) that were house cat sized, and eurypterids preceding them that were around 8 feet long. And keep in mind that there are constraints on land-dwelling arthropods both to do with support (what do you do when you moult at that size?) and respiration that animals with skeletons, lungs and pneumatic bones don't have to deal with. What's almost funny, given things like pygmy leaf chameleons and fingernail-sized frogs, is that modern birds don't get much smaller than they do. It all comes down to some combination of ignorance (and it's hard to know everything even in your own field, so no need for anyone to get superior or defensive where that's concerned at all) and incredulity. A lot of it is hard to imagine until you take the information that is available and spend some time actually imagining it.