LEDs are rated in lumens for total light and candelas for brightness in a given direction. More here: https://www.diodedynamics.com/info/research/led/led-brightness.html
An LED specification will also have a dispersion pattern, that is, how it is focused. This is somewhat analogous to viewing angle.
And now, a hacktastical solution.
There are commercial LED display panels that use smaller modules which can be combined to make displays of arbitrary size. They're driven with a video signal via a controller, so the display can be anything you like.
An example product (not a recommendation, only for illustrative purposes):
There are many vendors in this LED-based ‘digital signage’ space (many in China and Taiwan), so it's likely that you could find a low-priced source on Alibaba or suchlike.
Not as fun as making your own bespoke panel, yes, but it probably is more cost-effective (especially if you're making just one) and more aesthetically pleasing since the display can use any font and background.
As a bonus, you could potentially sell advertising on your clock and help defray the cost of building it.