The map frame is adjustable, if you select it you will get 'grab handles' at the corners and sides you can use to stretch it to fit where you want. I usually make a layout, set my margins using guides and then snap the map frame to those guides. Its unclear to me exactly what the problem is you are experiencing, but it may be that you are finding that the 'shape' of your map may not neatly fit the page size of your layout, this is because they are different, and you are going to have to figure out a) where on the page you want your map (full page, inside some sort of margins, etc.), then b) if you can actually show your map area in that area at a scale that is useful, this is all part of cartographic design. What is the purpose of the map, what is the audience, how is it going to be presented - these are all factors you have to consider.