You can change the thumbnail dimensions using the thumbWidth and thumbHeight configuration options.
For reference, a list of all Thumbnail Options can be found here.

When creating a gallery with JuiceboxBuilder-Pro, you can set the thumbnail dimensions on the 'Images' tab. (Just click the 'Change Sizes...' button in the 'Image Size' control panel to reveal the input boxes.)
Thumbnails will be generated at the specified dimensions and the thumbWidth and thumbHeight options will automatically be set in the gallery's 'config.xml' file accordingly.

Alternatively, just set the thumbWidth and thumbHeight options in your gallery's 'config.xml' file to whatever values you like (instructions on how to set configuration options can be found here) and, when the gallery is displayed, Juicebox-Pro will dynamically scale the thumbURL images to fill the chosen dimensions. (Please note that if your thumbURL images do not have the same aspect ratio as that set by the thumbWidth and thumbHeight options, cropping will occur.)

thumbsPosition is in the 'Thumbnails' section of the 'Customize' tab.
You need to be on the 'Images' tab (to the left of 'Customize') and then click the 'Change Sizes...' button in the 'Image Size' control panel (on the left side of the window). See the image attached.

If your have a full page gallery (i.e. it has dimensions of 100% x 100% and there is no other content on the web page), then the Expand Button will only be visible if you set useFullscreenExpand="TRUE" in JuiceboxBuilder-Pro's 'Customize -> Lite' section (for the gallery to expand to fill the user's entire screen). If useFullscreenExpand="FALSE" (the default value), then the gallery would expand to fill only the user's browser window (not the entire screen) and, as the full page gallery already fills the browser window, it would not be able to be expanded any larger so Juicebox hides the Expand Button.

If this does not explain what you are seeing, then please post the link to your gallery's web page so that I can see the problem for myself and hopefully help further. Thank you.

You're welcome.

Are you referring to the thumbnail dimensions or the Expand Button?

Thumbnail dimensions:
After clicking the 'Change Sizes...' button (shown in the screenshot attached to my last post), you should see a pop-up window (shown in the screenshot attached to this post). Change the 'Width' and 'Height' for the 'Thumbnails' and click 'Save'.
Then, when you 'Save' the gallery on the 'Publish' tab, thumbnails will be generated at the specified dimensions in the gallery's 'thumbs' folder and the thumbWidth and thumbHeight configuration options will automatically be written to the the gallery's 'config.xml' file with the correct values.
If you are experiencing difficulties with this, please let me know how far into the process you get and what seems to be the problem.

Expand Button:
As I mentioned previously, if you have a 100% x 100% gallery (which fills the browser window), then the Expand Button will be displayed on the gallery's Button Bar only if and useFullscreenExpand="TRUE" (otherwise the gallery would not be able to be expanded any larger than it already is).
You'll also need to make sure that showExpandButton="TRUE" (in JuiceboxBuilder-Pro's 'Customize -> Lite' section) and that buttonBarPosition ('Customize -> Button Bar') is set to a value other than NONE (so that the Button Bar is visible).
If you have trouble with a specific gallery, then please post the link to it so that I can see the problem for myself and hopefully help further.
Please also let me know what browser (or browsers) you see the problem in. Thank you.

OK, thanks.
If you still have problems, just post the link to your gallery and I'll take a look at it for you.

Thank you for the additional information and link to your gallery.

This is the source of the problem.

There are a few drawbacks to loading a Juicebox gallery into an iframe, one of which is that the Expand Button is disabled.
This is noted in the Using an External Gallery Folder support section (scroll down to "2) Using an iframe").

For full functionality (including the Expand Button), embed your gallery directly into your web page instead of loading it into an iframe using the baseUrl method from the link above (check out the "1) Using baseUrl" section).

As you are using Joomla, the Embedding in a Joomla Site instructions should help with the full procedure required.