I wanted to make the tiles smaller in the Platform Starter kit, because my game was going to be a mix between a robot game that I had in mind and Knytt Stories. I was first having trouble with doing this, but at last found the errors that I had, and because one person wants to see how to do this I have decided to write this tutorial.
First We make the graphics smaller.
A typical platform tile

64x48 in pixels
My tile

20x20 in pixels
All I did was made a tile of my choice the size of my choice. Once I got that bit done I just named in the BlockA0 – BlockB6, so I wouldn’t have to modify the LoadTile function in the Level.cs class. After that I did the following.
Editing the CodeChanged the Backbuffer widths and heights in the Platform.cs class.
CODE
//change the backbuffers so the map doesn't become to long to draw and out of proportion
private const int BackBufferWidth = 600;
private const int BackBufferHeight = 300;
Finally I changed the Tile Width and Height to the pixels of the graphics in the Tile.cs class.
CODE
public const int Width = 20;
public const int Height = 20;
If you compile the following above you should get your tiles to be the same size as the following picture below.

I haven’t finished making all graphics 20x20 as you can see the player is still the same size, but the rest are, and that is how you change the width, and height of the tiles using the Platform starter kit without messing up your game.