1) Is it possible on lightroom to choose the thumbnail size ? I know it is possible with "air" but i was wondering how to do it with lightroom directly
No. This is not possible. thumbWidth and thumbHeight are Pro configuration options and only Standard configuration options are available in the TTG SimpleViewer interface. This is noted on the Upgrading to SimpleViewer Pro TTG SimpleViewer support page.
Take note, the TTG SimpleViewer engine will only ever offer options permitted by the SimpleViewer free version. To customize Pro galleries created with the TTG SimpleViewer engine, open the published gallery with the svBuilder-Pro application.
2) I'm building a responsive gallery
Thumbnails in a SimpleViewer gallery do not scale dynamically and the number of thumbRows and thumbColumns always remain constant. This cannot be changed.
If you need the number of thumbnails in your gallery to change dynamically, then you might like to take a look at another one of our products - Juicebox-Pro. Juicebox-Pro has maxThumbRows and maxThumbColumns configuration options which set the maximum number of thumbnail rows and columns that will be displayed. The actual number of rows and columns which will be displayed is dynamically calculated from the size of the screen and the position and size of the thumbnails themselves.
Take a look at this sample Juicebox-Pro gallery and reduce the height of the browser window. You will notice that the number of thumbnail rows reduces as the height of the browser window reduces.
Likewise, in this sample Juicebox-Pro gallery, reduce the width of the browser window and you will notice that the number of thumbnail columns reduces as the width of the browser window reduces.
For more information on the differences between SimpleViewer and Juicebox, please see this SimpleViewer news article.
SimpleViewer Support Team