When you have several pictures or objects on the slide it can look neater if they are the same size or similar, by that I mean that if you have a lot of objects across horizontally but some have more information in them then others it might look better to have them the same height but they can be different widths.
If you have several pictures running horizontally or vertically it looks better if they are the same size especially if they are in a coloured band. It goes without saying that they should be spaced evenly, which we have covered in a previous post.
When you select a picture you can change the size by going into Format Picture (right mouse click). There is height and width available but there is also a tick box for aspect ratio. If not ticked this will allow you to squish or stretch the picture out of proportion, so you could double the height without doubling the width.
You can also select multiple pictures, click on the first picture then hold the shift key down and click on the remaining pictures, then let go of the shift key. Then either right mouse click or if you are using PowerPoint 2007 or 2010 you can use the picture tab at the top and change the height or width, if these are blank it just means that the selected objects are not all the same height or width so it does not show any number.
When I have quite a lot of slides I sometimes want all the pictures to be the same size of slides, so I use the height and width to adjust them. If you find that the picture is skewed (ie the height changes but not the width of the picture) then press the right angle button in the bottom right of the ribbon section to open the size window. Then tick the aspect ratio tick box so that the height will always change relative to the width and vice versa.