You can make the thumbnails larger with SimpleViewer but only for the Flash Player version of the gallery.
When creating a gallery with svBuilder-Pro, click the 'Change...' button in the 'Image Size' control panel on the 'Images' tab and choose the thumbnail dimensions you'd like to use. Thumbnail images will be generated at the specified dimensions and the thumbWidth and thumbHeight configuration options will be set accordingly in the gallery's XML file. The thumbnails will then be displayed at the specified dimensions in the gallery but only when the gallery is displayed by the Flash Player. If the gallery is displayed by the Mobile Player (in browsers which do not have Adobe Flash Player installed and enabled), then the thumbnails will be displayed at the default dimensions of 75 x 75. The Mobile Player was designed as a fallback mechanism (so that gallery images can be viewed on non-Flash-enabled devices) rather than a fully-featured image viewer within itself and, as such, supports far fewer configuration options than the Flash Player. The Mobile Player Options can be found here.)
Right now the galleries have the tiny thumbnails and once you click on that they change to the larger image.
It sounds like, in order to display your Flickr images, you might have forced your gallery to be displayed by the Mobile Player (by deselecting the 'Use Flash' checkbox in svBuilder-Pro's 'Customize -> Embed Options' section). If this is the case, then, unfortunately, it is not possible to change the size of the thumbnails.
Perhaps the best solution would be to switch to our HTML 5 image viewer, Juicebox and, as a SimpleViewer-Pro user, you are eligible for a 30% discount off the cost of Juicebox-Pro. Please see this blog entry for details.
Juicebox-Pro supports Flickr, has many more configuration options than the Mobile Player within SimpleViewer (such as the ability to change the thumbnail dimensions) and does not use Adobe Flash Player at all.
It is also possible to convert a SimpleViewer gallery to a Juicebox gallery by following the Converting a SimpleViewer Gallery instructions.
I hope this helps to clarify things.
SimpleViewer Support Team