Assuming you're using version 10, just go to Share/Computer/AVCHD and select the 1920x1080 option and you're there.
The file may not be exactly the same size as the original. Your camcorder may not use the exact brand of H.264 codec and settings as the program. But it will be virtually identical.
Although being virtually identical has very limited uses. What do you plan to do with this video? If you want to burn it to a disc, put it on a web site, load it on an iPad or whatever, there are various settings you should be using for each of those situations.
That's the whole point of AVC video. It can produce a high-quality file in a variety of sizes and quality levels to meet the unique needs of your delivery system.
Oh, forgot to tell you that I use Elements 9.
The output will be a full HD file both to view on a computer and to upload to my Vimeo Plus account.