I think 6" digits are fine. I time professionally and use Electronumerics Raceclocks. 6" digits double sided. Honestly I prefer the traditional style and not the LED, but I also have an LED version which is a bit cheaper and lighter. I use it as a backup clockand also use it in dark and in poor weather conditions. In direct sunlight I still think the traditional neon yellow painted flip digits are more visible.
Double sided is more expensive and preferred, but depending on what you're using it for, you may be able to get away with single sided. For professional timing I don't think single sided is adequate.
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