Scanning A4 & A3 without having to save seperately

I have Acrobat Pro 11.

In Acrobat 7 that i used to have, when scanning different size pages i could scan A4, then the prompt for scanning more pages would come up, when i used to say yes I would get the option to change the scan size to A3.....with 11 i don't get this option to change size. Is there a setting that i am missing? I have documents that have a mixture of sizes that i scan & it is very frustrating that i have to scan 1 part, save it, then scan another size & save it, & then have to do this repeatedly.......and then have to combine all the scans into the correct order. I have a fujitsu fi-5750C scanner.