How can i create different sizes for each page in 1 file, in InDesign?

There are a couples ways to do this, none, however, are as simple as choosing different page sizes within the same document.

The first is to use the Book feature to link the attributes of multiple files. You can synchronize style sheets, colours, etc. and at least the odd-sized pages will be part of the same Book and easier to keep organized. Not an ideal situation, but workable.

Another option is DTP's Page Control plug-in, which gives you just that: total page control. There's more information available at their website http://www.dtptools.com/product.asp?id=pcid.

Also if you have Illustrator CS4, it's a bit easier as it lets you create pages with different sized artboards or "pages"