Well since you only made your first room yesterday (hopefully you figured out how to spawn now) you might want to hold off on the advanced scripting stuff until youve got more of the basics down and understand how thigns work better. Learning for example how to make a door in a lit room and finishing a simple working map will teach you a lot more a lot faster than diving straight into the kind of insanity laid out in the thread you linked to.
What you are trying to do might seem simple but I think it is going to be a lot more complicated to get it working how you want it and working out the issues that will come up. But since I just played with this and it is kind of hilarious I will give you a little simple help,
Put your medium ammo box in the game, go into properties and click Outputs
Press Add so a new field appears and click the pulldown for My output named, change it to OnPLayerTouch
In "target entities named..." type in !activator (it will appear in red text but thats ok)
Via this input type in SetModelScale
"With a paramater Override..." type in how you want it to scale the player. Enter 2 to make him twice as big
Compile your map and try it. Remember you will need to fire off some ammo before you can collect the box because you cant pickup a box with full ammo (this is an issue) and you can only scale up once, you will need to work out a system to detect when a player is scaled up by two so you can scale him up again the next time he collects it. I'm not sure what happens when you die or change class, you will need to work out how to scale the player back down like it says in the tutorial, which is a whole new complicated thing to get in to, not to mention what will happen when you are running in small spaces and things like that.
Basically you are not just trying to learn to run before you can walk, you are trying to learn to pull off a double backwards somersault with a half twist before you can walk.
Taunt and admire your monstrous player, it is pretty hilarious.