How do you properly size a speaker?

I have a 40x2 watt 4 or 8 ohm tube amp. I am very inexperianced when it comes to proper set up. I literally just bought the cheapest vintage speakers I could find with a name I knew they had no ratings i had to find that info later. I noticed some speakers have a span for wattage like 30 to 75 watts for example. I bought a pair of cerwin vega L7s for 50$ I saw that they are rated 75watt and 8ohm, there is no spectrum on the wattage that I could find. Does that mean I am running them under powered with a 40 watt amp? These are like smaller monitor speakers, would it make a difference to run the bigger floor speakers? Seems like I have seen bigger floor speakers with bigger woofers and more tweeters with wattage ratings from 30 and up? Correct me if I'm wrong I'm just curios because I will be buying a home soon and I was thinking about getting some bigger speakers when I have the room for them.