This sounds interesting. Would there be a limit to how big they grow?

what?

In the animation they all grew. Could it be possible for them to grow more?

He's talking about the dick and boob size and nuts

There's a lot of factors in deciding how you want to present your content. Consider first writing down what you want ideawise and then work from there. The engine that you want to use will be important, but having a cohesive idea and how you want to apply it will help you determine what sort of engine you might need to put it down.

There's a few things to consider:
- How do you want to distribute the game? Download? Web browser? Plug-in for web browser?
- What do you want the engine to do? (Think of 2d or 3d, importing and rendering models, handling animations, user input)
- What kind of game do you want to make? (Linear story? Let the player choose (branching)?)

well if you do plan on making this game, which certainly sounds interesting, I request that you don't hold back on characters. Shade the echidna doesn't get enough love.

A game where you can make Sonic girl boobs, dicks and balls grow? I'd love to play that.

Regardless of which engine you choose, the first thing you'll need to keep in mind is that game development is a work of dedication. It'll take time to learn, time to make, time to refine and time to perfect. With each step taking longer than the last for most.

But don't worry. Once you get started, it's really fun. x3

It's unbelievably how gratifying it is to see things work according to the rules you set for them, and even better when they don't, and you figure out why. :P

Will this upcoming game you'll be working on a online game or no?

Hey will you use the werehog for sonic

What type of game it'd be?
A one where you click certain parts of characters to make something happen or a one where you make choices to have different scenes?

Maybe take a look at unreal 4, seams to have alot of flexibility if you intended on using the same animation style.
It can depend alot on the type of game you had in mind though, heck it could be something as simple as a high end RPG maker game but with a load of cut scenes added, that would still take hundreds or even thousands of hours of work though.