One digit is 190mm long, 110mm wide and depending on what connection cables you use about 12-15mm high and you can combine as many of them as you want. The only limit is the memory of your microcontroller and the refreshrate you want to achieve.
I combined four of them as you can see on the pictures.
It is designed to fit 7 "8 Pixel WS2812b Led-boards" (1 per segment) and therefore can display every color you like and you can break down animations to a single pixel.
I sourced them from aliexpress. The link below links to an offer of 10 pieces, feel free to compare prices and find other boards. They are available on black and white pcb's and keep in mind that it is never wrong to have some spare parts ;)
10pcs 8 channel WS2812 5050 RGB LED board
Best filament for the diffusor parts is transparent one. It also works with white but this will reduce the brightness of the display. (but result in a better diffused display)
For indoor use the brightness is more than enough as long you do not use black filament ;)
You may noticed... the diffusor is upside down so you have to rotate it in your slicer.
Print the SevenSegPanel once and, if you want, 7 of the SevenSegDiffusor to get one complete digit.
If you have printed the parts it is time to hotglue (a very small drop of glue is enough and if you end up having to change a module it makes everything much easier ;) ) the led boards into the frame. No need to worry about the direction as due to the printed pins it is not possible to mount them the wrong way. (of course you should check that the led is facing upwards ;) )
Now turn the display around and connect the modules in a serial manner together. Have a look at the picture to see how i have done this.
All you have to do now is to connect multiple displays together (if you built more than one) and connect the input to some sort of microcontroller.
I may add some code later that does the displaying part but i am sure you can figure this out easy by yourself. There are plenty of librarys out there i think.