What size of monitor would be needed to show 4k?

4K usually means a display is 3840 x 2160 pixels. 

However, those pixels can be ANY size, so your 

question is nonsensical. With small enough pixels 

I can put the whole thing on a 10 inch screen, or 

with larger pixels I can put it on a 100 inch screen. 

Trying to guess what you are really asking, if for 

example you are wanting to look at an image taken 

with a 10 megapixel camera, the image would be 

around 3872 x 2592 pixels, so at full resolution, 

you would not be able to see all of it on a 4K 

display (around 30 pixels missing on the sides 

and around 430 pixels missing on top/bottom). 

So if you are average and healthy, you should have a visual acuity of around 20/15 at which point the need for a higher resolution is even greater. At that visual acuity, for anything larger than a 15.5 inch screen you would ideally want to have a 4K screen.

four feet long1 for all the pimxels :) hope this helps!

resolution and physical size are not related.

a 4k image can be 1x1 or 100x100 

if you type this on the search bar it might tell you