So yeah I started editing maps today, and I learned cool stuff. But the problem that bothers me is how to make map bigger in size, and how to make custom objects that mice can push

Hey,Your answer is:

to make it horizontal
<C><P L="4800"/><Z><S /><D /><O /></Z></C>
You just need to add the word L that stands for Length ,after the <P

to make it vertical
<C><P H="800"/><Z><S /><D /><O /></Z></C>
You just need to add the word H that stands for Height,after the <P

Two of them:
<C><P H="800" L="4800" /><Z><S /><D /><O /></Z></C>

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And i am not really sure of the Custom objects ,since i am not a pro map maker
Anyway next time if you have any questions about the map editor,coding a map please put your question in Here

I think the thing you're looking for is how to make a scrolling map. You can find it in this thread
For other tips & tricks you could possibly use this thread

Good luck!

Thank you (both of you)