To change the items size, see the constants at the top of MyLauncherView.m. This will be made easier in a later release.
At the present time, there isn't any way to put the items in specific positions. MyLauncher stores the items in a standard NSArray and simply loops through the array adding the items in the order they appear to the page. These array's then go inside another NSArray called pages, one NSArray per page. So you can put them on different pages if you want (providing no middle pages are empty).
By different positions, do you mean you want to leave a few slots empty where items would fit, or that you want to customise the x/y position of a particular item for fine-grained positioning?