What the numbers mean
As nothing with RC helicopters is easy, this is also not so simple and it has got a history. Traditionally the measuring in modelling is expressed somehow. The easiest way is the scaling from the reality. This problem is really simple with static models, like 1:35, 1:72, etc... It expresses that 1 unit is 35 or 72 times bigger in real life. When this is not so accurate - it can happen - something else can express the dimensions.
Like in railway modelling: N, TT, H0, 00, 0, G. Why? For example, the really popular 00 size in UK is not exactly 1:72. The gauge is 16.5 mm, which is excatly the same as the H0 gauge, however the nominal scale of H0 is 1:87, so there is a typical distortion with is common at all manufacturers working in same scale. There are good reasons, why the scale is not exact, but railway modelling is a different world and this is an RC helicopter website at the end.
Well, there is a problem how to measure these helicopters. As long as a scale model of a real one is measured, life is easy. The scale can be expressed by ratio. But what if a performance model has to be categorised? What's going to be the comparison base? A Mil Mi-26 with its 32 mttres rotor diameter? Or a Robinson R22, which has gat smaller rotor diameter, then the size of the tail rotor on a Halo? By the time and appearance of performance models, this scaling wasn't good enough anymore. another solution has been required.
Therefore these helicopters with their nitro glow engines have got measured by their engine sizes. 30, 50, 90. The meaning of these numbers is the engine cylinder capacity in hundredths of a cubic inch. These figures described the power and the size of the helicopter quite accurately.
Then electric helicopters were born in bigger sizes. There was the demand to categorise them by their dimensions. The engine size wasn't useful anymore, because there wasn't engine. If I'm right Align has figured out the similar system based on electric motors. Just instead of using the cylinder capacity, theyv've began to use the inner length of motor shaft in tenths of milimetres. Therefore the 500 size helicopter uses motors with 50 mm length motorshaft - excluding the outer part of that shaft.
Other factories like Thunder Tiger have decided to use the length of main blade. That's why a Mini Titan 325 is as big as an Align T-Rex 450. And this is the system in use nowadays - except by Align. Align is still using a weird combination of these two systems, where the motor size based measuring does not express the exact size, but expresses the location of the model on their product list. And this is why a SAB Goblin 380 is bigger, than an Align T-Rex 450.
And if this is not complicated enough, even the length of the blade is not so exact. There are variations inbetween limits. For example a SAB Goblin 380 Kyle Stacy edition flies with 360 mm long blades, whilst original flies with 380 mm length blades. At the end of the day: we still have no exact measuring just few different informal systems expressing the size category more or less.