A presentation with 20, 200kB images won't be much larger than 4MB which is a perfectly reasonable size for a PowerPoint file. If it looks like the sum of the sizes of all your images is approaching 10MB you really need to start thinking about shrinking some or all of the images. In the end the maximum size for a PowerPoint file depends on the memory setup of the machine you're using and how long you're willing to wait for things to load. You can increase and decrease either of these to increase or decrease the file size. Lower color depth gives you less accurate colors.
Resolution is measured in pixels.
For instance one standard size for a computer screen is 640 pixels by 480 pixels. Note that by varying the size of the pixels one can change the size of the image without increasing the resolution. This can cause some confusion since the pixels that make up the image on the actual computer screen are fixed in size. If you've made the pixels in you image file large each one may be represented by several actual pixels on your computer display. Only changing the resolution will change the file size.