There are a few settings for image sizes.
Thumbnail and large image.
AdminCP => vBGallery -> Modify Settings => Uploading Options
- Thumb Size
Large Size: *
AdminCP => vBGallery => Edit Filetypes
- Max Width
- Max Height
*Note: This is the default size set for each category created. There is also optional setting to change the size of the large image on each category setting page.