(assuming windows) Right click the jpeg or tiff or whatever it is, click properties, and look at the details tab. That should give you the width and height in pixels.
Divide the print size into the pixels to give you an approximate pixels per inch (ppi).
e.g. 3000 x 2000 pixels and you want to print it 20x16". 3000/20 = 150 & 2000/16 = 125. Assuming you're cropping to the paper, use the worst resolution, in this case 125ppi.
Depending on your (viewer's) eyesight, pretty much anything over about 120ppi will be fine. Those files sizes look small to me and will probably be in the 1000 x 750 pixel size which for a 20" print is about 50ppi. That will look fine at about 6 feet because they eye won't be able to tell the difference but closer, you'll start to see issues. If the sizes you've given are for the overall print, then it might be a problem as people will want to get close to see the detail in the collage. Their eyesight will also play a part, and what you call "looking poor" is subjective.
I'd aim for 120ppi.
The other option is to crop an A4 sized bit out of the image, and get that printed as a tester before ordering anything expensive.
