Akatsukami's discussion about the radius of a proton and the radius of the observable universe is not quite dead on. A proton is just a cloud of quarks; its "radius" depends, at least in part on the energy level of the proton. Again, the "radius" of the observable universe is not a truly fixed number, but it is harder to pin down than the radius of a proton.
Scientists tend to translate enormously large numbers, or enormously small numbers to something else. Astronomers use "light year" to represent an enormous distance. But, if you think about it, a "year" is an enormously flaky number. What is a "year" in an absolute sense? It is the time it requires us to complete one rotation around our sun. However, we seem to have trouble measuring a "year!" From time to time we have to add a "leap second" to our year because we can now measure time more precisely. Or take it from another perspective. In 1967 I celebrated the start of the year near Philadelphia, Pennsylvania. In 1968 I celebrated the start of the year near Chia Yi, in Taiwan. My 1967 was somewhat shorter. Similarly, I celebrated the start of the year 1998 in Moscow, Russia; my year was somewhat shorter than if I had celebrated the start of year 1998 near Philadelphia. By 1999 I was back near Philadelphia, so my 1998 was 8 hours longer. Of course, over two years my two years stayed the same.
Physics uses the term "electron volt" as a way to measure energy in sub-atomic reactions. By any human standard an electron volt is an extremely tiny amount of energy. The energy to light the display on my laptop is probably the equivalent of billions of electron volts.
Now 1.2345678E-10 is a much different value that 1.2345678E10. The difference, of course, is in the exponent. 1.2345678e10 - 1.2345678e-10 is still 1.2345678e10.
8 digits is about all you can do in 32-bit IEEE floating point 1.2345678 generates 3F9E0651, 1.2345679 generates 3F9E0652, 1.23456781 generates 3F9E0651, as does 1.23456782.