The dojo-based Esri library can have trouble loading off-screen maps. I had to use this hack to get the map to size correctly
Reveal.addEventListener( 'slidechanged', function( event ) {
var frames = $(event.currentSlide).find("iframe");
if(frames && frames[0]) {
if(frames[0].src.indexOf("_dojo_") > -1) {
frames[0].contentDocument.location.reload(true);
console.log('reloaded frame');
}
}
} );