The dimension of the thumbs can be changed in relation with the view mode (default, and enlarged/preview mode).
From the included themes, the only one that sets an explicit dimension on your thumbs, in the default view, is the “square” theme. You can find this value in your theme’s CSS file. Look for “.accordiongallery.square section .accordiongallery-thumbs a img”. As I said, this is only available for the “square” theme.
For the other themes, you can set the dimensions of the thumbs in the markup, setting the width and height attributes on the img tag.
On the other hand, in the enlarged/preview mode, most of the included themes set the dimension of the thumbs at the bottom of the column, in the CSS file. Look for “.accordiongallery.square .accordiongallery-preview-scroller a img” (this is for the square theme, for the other themes change the “square” value to the name of your theme), and change the width and height properties.