I've taken a quick look at a Bloxx gallery and one of its features is that it is flexible. This is actually a good thing, because it means that people can view the gallery on different devices (computers, tablets, mobiles etc.) without having to scroll horizontally.
It's possible that you can force the gallery to have a fixed width by editing a CSS file, and one of my colleagues could advise you on how to do this, but a fixed-width gallery is not very mobile-friendly - so you would risk alienating visitors who use a mobile device to visit your website. That's actually the majority of website visitors these days.
An alternative to making the gallery have a fixed width might be to give it a maximum width (again, this would probably require a CSS edit). This would allow the gallery to be mobile-friendly, but would restrict the number of images that are displayed horizontally on desktops and/or tablets.