Being new here, what is all the numbers mean on plane sizes? 40,60,90 140??? What exactly does this all mean. Thanks.

Numbers refer to recommended engine sizes. 40 is 0.40 cubic inches displacement, for example. Engine weight and power is related to displacement, but not as much as you might think.

So the size of the plane itself is judged by how big the recommended engine size is? As you stated, is that the maximum size engine?

Not necessarily. Most kits specify a range of engine size. For example: .40 - .60 2 stroke. .46 - .72 4 stroke.
Some just use one engine size, and or just one type of engine.

This could mean one or a number of things, among which are:

1.) the minimum and maximum sizes recommended for adequate performance.
2.) The maximum size recommended for airframe structural intergrity under normal flying use.
3.) A WAG by the distributor that would produce the most sales.
4.) A known engine size that will work for the size of the airframe.

Big gassers often use such as 50cc or such -- 26 -- 32 which is C Centemeters displacement. Just check out the engine ads.
Hey, I don't know diddly about electric stuff so those experts can check in on those varmints.

Now not in those engine sizes are as Jim stated, engine size doesn't always define power output. A Dub Jett .90 is in another world when compared to a Super Tigre .90. Kind of like comparing a Diesel to Ignition in a car. Many other things prevail over simply cubic inches, however when not totally in the know, Cu. Inches prevail!

One item of safety; When using ARFs, really check out the firewall structure. Many are poorly designed and inadequately reinforced for active use. Extra reinforcement will save you some heartache in a loss of machine or even fingers when the engine departs the airplane. -HIT happens.

You'll figure it all out and have fun doing so.