I'll try to answer to the best of my limited knowledge.
The first thing to consider when resizing is whether you have any other houses on the account. If you have grandfathered houses they will go condemned. If you have a condemned house (maybe you just placed a larger plot than you currently have and want to resize) it's likely you won't be able to replace on the resize lot if you haven't demo'd the house before resizing. Before resizing you should have only the house you plan to resize on your account.
Yes, you will have to redo all containers and rebuild from scratch. Resizing is actually a Demolish of your house, while placing a hold on the land where your footprint is. It is an actual demolish, and that's why other houses on the account can be a consideration, whether grandfathered or in demolition mode.
Yes, Since you're demolishing and placing a new house you will have to rebuild/redeco your new house.
Friends and Co-owners will be reset.
You cannot resize a house with rental vendors. Resizing a house is like a demolition, so your own vendors will be lost. Although normally you'd expect them to go into the house sign, since it's a demolition, it's like an IDOC... all items in the home will be dropped under the house sign and freely lootable. You must clear your home of all contents prior to your resize, including vendors and their contents.
Hopefully someone else will verify my input, but I believe it is accurate. Something to note, the resize option can bypass the normal 7-day waiting period for placing a home. You must wait one hour after placing to resize. When you resize you have one hour to place on your plot and you have only one placement per resize, so know what your maximum size is before resizing, or at least build big and work your way down. You can resize your plot every hour, however, so if you do not place exactly what you needed you can resize again an hour later.
The description of resizing when it came out in the publish stated it guaranteed placement by any character on the account for one hour. The gaurantee I take with a grain of salt. Once you resize your house IS demolished and gone. If you can place a new house on the plot within the hour you've succeeded. There are some things players can do to block placement (drop static objects, cast gates, kill pets) that could theoretically block placement for a time. To truly gaurantee the plot remains on the account I wish that after one hour, if nothing has been placed, that your original plot would auto-place there - or perhaps a box to check on the housing gump 'replace original plot', in case for whatever reason you were unable to place your new home, but that's just wishing.
Good Luck.
