Open up the 'C:\Program Files\Adobe\Adobe Photoshop Lightroom 4.1\Shared\webengines\autoviewer.lrwebengine\galleryInfo.lrweb' file in a plain text editor.
Change lines 25-28 inclusive from:
[ "photoSizes.large.width" ] = 640,
[ "photoSizes.large.height" ] = function()
[ "photoSizes.large.width" ] = 9999,
[ "photoSizes.large.height" ] = 600,
... and change lines 32-33 inclusive from:
[ "photoSizes.large.maxWidth" ] = "1500",
[ "photoSizes.large.maxHeight" ] = "1500",
[ "photoSizes.large.maxWidth" ] = "9999",
[ "photoSizes.large.maxHeight" ] = "600",
Essentially, change the height and maxHeight to the height you'd like all your images to be and set the width and maxWidth to a very high value.
This will ensure that all your images are resized to the same height.
However, bear in mind that AutoViewer dynamically resizes images depending on the size and shape of the user's browser window and will, therefore, only display the landscape and portrait images with the same heights if the browser window is of a certain aspect ratio.
In order to override the dynamic resizing of images, you would have to purchase AutoViewer-Pro and use the enableImageDownScaling and enableImageUpScaling Pro Options.
SimpleViewer Support Team