The searchbar doesn't size at all/sizes weired

It's very likely that you've got either a "bad" theme (haven't seen one in ages though) or some left-overs in your userChrome.css. Try removing any searchbar related stuff from that file in your profile folder. If it doesn't work, the file doesn't exist or you don't know what to do, contact me and provide the following: Autosizer version, Firefox version, which Theme and in which version plus a link where I can get that theme. I'm sure we can fix it.

How to change the default engine Autosizer switches to?

Open a browser window where the searchbar is visible and click on the search engine's icon (e.g. Google icon). Choose "Manage Search Engines..." from the menu (lowest entry) to open the manager. Select your prefered default engine and move it to the top by clicking the "Move Up" button or simply by dragging it to the top. See Screenshot

I updated from 1.5 to 2.0 and now Autosizer doesn't work anymore.

Your Autosizer installation is outdated. Goto the "Description" page and click on the left install button.

The auto complete drop down doesn't display any text.

This was fixed in 1.2 and up, please update. You can adjust the suggestion width by going into Autosizer's option dialog ("Advanced" button in Assistant) and changing the "Suggestions Width" value. Restart Firefox after you did so and the popup will at least have the specified width.
Alternatively you can use any other mode than "size incrementally" to fix this problem.

Some letters get hidden when I type although the searchbar isn't at its maximum width (applies to input helpers as well).

This was mostly an issue in versions prior to 1.1.1, however it may now reappear, if you use very "wide" letters or use an input helper tool (especially Asian languages). Autosizer actually auto sizes in advance, making room for another letter even though you didn't type it yet. If your letter is wider than the room Autosizer made before, letters will (partly) get hidden on the other side. Increase the offset to fix this. The optimal offset width is the width of the widest letter in px minus 9. For example, for most western languages the capital W is the widest letter, on my system it's 9 pixels in width. Therefore the optimal offset would be 0 pixels.

There are red bars on each side of the searchbar

• Open the Add-Ons Manager
• Double Click on "Searchbar Autosizer" to open the Assistant (if the option dialog opens, click "Assistant")
• Choose any option and click "Next"
• Click "Cancel"
• Red Bars should be gone

Will you make it work with Google Toolbar?

Already done, see "Other versions" page. It'll never make into the offical version though because there's no easy solution combining two or more searchbars. Plus, Google Toolbar sucks and its functionality can be replaced by better extensions.

Will you make an "URLbar Autosizer"?

This has been requested several times, although I don't get the point. URLs may be very long and the interface would change every tab switch... in case you still want it you can get it here.