It's a much talked about issue; the pixel vs the inch, print vs online. Designers are separated into groups when it comes to their opinion on what to use for web design, or what designers out there are using as far as applications. Some are solely using Photoshop for their creations, some in the Photoshop/InDesign group and some in Photoshop/InDesign/Illustrator+ group. Some argue the Photoshop text engine is far inferior than that of InDesign's text engine, and you should be using Photoshop for your imaging and InDesign for typography, and use illustrator to it's full abilities with both. Indesign offers great choices for exporting too —You're game if you have this recipe. They say anyhow.
If you are going to use InDesign to create your 72ppi work, when you open a new document it doesn't offer a pixel as a unit of measurement. So some would have to figure out the size in inches from pixels.
I have a quick and easy way to convert your pixels into inches, providing you have Photoshop open. Why not make Photoshop calculate it?
Go to Image on the top menu bar and select image size.
Next, put in your web image dimensions in the box. I'm going to change this to 1000 x 1000 pixels.
Next you see that Photoshop will calculate your web image dimensions from pixels to inches. Record your inches measurement.
Below you see in your InDesign you just put in your measurements and you are ready. Hope this helps!