What watch face shows heart rate larger and constant

Look at the specs for X-Large!

It says that the complication will fill the screen 😳

"Constant" is going to be the only remaining issue. I doubt that it's possible, your battery would likely die rapidly.

Honestly, I don't think that there is one that is specific for that. Heart rate seems to only be a complication that is available for a lot of the different faces.

Here's a useful link to all of the faces and it shows what complications are available for each:

Apple Watch faces and their features - Apple Support

Wait! Look at the picture for Modular Duo (from that link I shared 👆).

They have the heart rate set as the middle complication.

So... I suppose that there are a lot of watch faces that allow you to put a complication in the middle 💡