You are confusing MB (megabytes) with dpi (dots per inch). The size of your renderings in MB is determined by the horizontal and vertical dimensions and the resolution in pixels per inch.
In the Render dialog you can specify the dimensions in the middle of the dialog, on the right side. To change the dpi to something other than the default of 72 you need to first change the Units to something other than pixels. Once you've done that the dpi field is made active so you can change it as well.