How can I determine if a USB-C power bank will actually charge my MacBook, before buying it and testing it, based on the MacBook specs and the power bank specs?
Apple uses USB-PD for providing power over USB-C so the charger must support USB-PD. The chargers Apple supplies with their laptops were built to provide enough power to keep the computer running at 100% load indefinitely. Or at least that's the case most of the time, I do recall reading complaints of people running long overnight CPU intensive processes to discover that their MacBook went to sleep from burning through the battery while plugged in. That's not how most people use their laptops though.
I discovered that I could run my MacBook Pro indefinitely from a 27 watt power supply when using it for web surfing and similar light duty work. This was a 12 volt automotive USB-C charger that I ran from a sealed lead acid battery during an extended power outage at my home. The computer came with a 61 watt charger, making the 27 watt charger half the size of the original.
Note that a MacBook will likely be able to take more power than what the included power supply can provide. This is proven by my MacBook Pro happily taking more than 61 watts from a 87 watt charger I bought as a spare.
USB-C chargers will provide 5 volts as well as one or more other voltages, those being 9, 15, and 20 volts. (Those are nominal, the actual listed voltages could be plus or minus by less than a volt.) As far as I know Apple laptops will charge from any of those four voltages. I charged my MacBook Pro from a 5 volt 12 watt USB charger before, v e r y s l o w l y.
My experience shows that, with a sample size of one, that a MacBook Pro will charge from any of the four standard voltages from USB-PD/USB-C. USB-PD was designed to keep charging amperage between about 1 to 3 amps, going into the 3 to 5 amp range only at 20 volts. Selecting the power output capability of the battery pack will depend on how quickly you want to charge your computer. Making notes, mentally or otherwise, on how long it takes for your laptop to charge and comparing that to your needs and preferences should guide you on what output voltage to look for.
Because USB-PD specifies a set range for current and voltage the power rating will give some indication of the voltage, and vice versa. A 27 watt charger like I have will almost definitely provide a maximum of 9 volts. 45 watts likely means 15 volts maximum. Anything above 45 watts always means 20 volts.
And what is a "reasonable" capacity for a power bank used on a Mac (or laptop in general)?
This again depends on your experience, needs, and preferences. Use the capacity of the battery in your laptop and how many hours of use you get out of it as guidance on how big of a power bank you need. The capacity of the battery can be found in the System Information app under the Power tab. Take the mAh, divide that by 1000, and the mV, also divided by 1000, and multiply them together to find the watt-hours (Wh) of your battery. That will help guide the size of power bank you will want.
I hate that power banks are often advertised with their mAh ratings as that is useless without knowing the voltage. Find the Wh capacity of the power bank as that is the number you will care about, the mAh rating is meaningless and should be ignored. Evaluate how many charge cycles you want or need from your power bank and compare that to the capacity of the battery in your laptop. If the battery lasts about 4 hours for you on what you consider typical usage, and you think you need 12 hours of use, then get a power bank that is double the size in Wh of your laptop. That gives 4 hours on the original laptop charge, then two charge cycles from the power bank to get to 12 hours.
So, look for USB-PD support on the power bank above everything else. Without USB-PD the power bank will charge your laptop very slowly at 5 volts or not charge it at all. I know I spent a lot of words on voltages but it's not terribly important, I gave that detail to explain why it is not important. It's watts that will tell you how fast it will charge your laptop. The more watts the less time it will take to charge. 12 watts is likely the absolute minimum, anything below that and your laptop will likely refuse to charge. Don't be concerned about the power bank being more powerful than the charger that came with your computer, so long as it complies with USB-PD then it will be safe. Look for capacity in watt-hours, amp-hours and milliamp-hours mean nothing. Compare the Wh of the power bank to the Wh of the battery in your laptop to estimate how many hours of usage the power bank buys you.