Apologies for the tardy reply. I set-up 4 example galleries and then tested them on an iPhone 5c running IOS 7.4 and an iPhone 6 running IOS 8.2 and then IOS 8.4. I'll leave the example galleries up for 10 days. Let me know if you need longer.

So, the example galleries:

1. http://www.brianwebster.uk/index/juicebox-one
The viewport metatag is enabled
autoPlayOnLoad='False'
showSmallThumbsOnLoad='True'

2. http://www.brianwebster.uk/index/juicebox-two
The viewport metatag is enabled
autoPlayOnLoad='true'
showSmallThumbsOnLoad='False'

3. http://www.brianwebster.uk/index/juicebox-three
The viewport metatag is disabled
autoPlayOnLoad='False'
showSmallThumbsOnLoad='True'

4. http://www.brianwebster.uk/index/juicebox-four
The viewport metatag is disabled
autoPlayOnLoad='true'
showSmallThumbsOnLoad='False'

Results:

Iphone 5c and IOS 7.2 displayed all the galleries without error.

Iphone 6 IOS 8.2/8.4 displayed galleries 1 to 3 without error. It failed to display gallery 4 correctly. The buttons don't display in portrait. Tapping the screen does not unhide them. You have to turn the phone to landscape before they are displayed. Once the buttons are displayed they work as expected.

Am I likely to encounter problems if I leave the viewport metatag enabled?

Thank you.

Thank you. If  small screen mode is set, I know reset the icon size (20px) and containers to the appropriate values. This seems to have done the trick.

Yes, if one is using a lot of buttons, they will wrap on mobile devices. It's not that I want them to scale depending on the number of buttons, I just want to change the size of the buttons in small screen mode because they are too large.

(I expected them to be smaller in small screen mode is all. I appreciate that I didn't explain myself very well.)

I've used the following code to try and solve this.

var jb = new juicebox({
    containerId: '@Model.Container'
});
jb.onInitComplete = function (e) {
    var mode = jb.getScreenMode();
    if (mode == 'SMALL') {
        $(".jb-bb-button").css("font-size", "16px");
    }
};

The problem with this is, the buttons initially display at the preset size of 20px before resizing. Is there a better way of applying the resize so this doesn't happen?

Thank you for the speedy answer. This is the kind of solution I was hoping for. I will give it a try and get back to you.