The length of DNA is not what's important; it's the information in the DNA which matters. This DNA information directs the processes of change which we can observe.
Correct. Most new DNA is 'junk' DNA, an artifact of the imperfect replication process that most DNA based life exhibits.
If DNA gets longer, then you have to ask where did it come from? Did this "extra" DNA spontaneously come into existence, or is it the explicit product of former DNA?
It is most often due to an extra copy of a gene due to transcription error, or even an extra base pair created during strand slippage. Since most DNA is 'junk' DNA (i.e. doesn't code for anything) it usually doesn't do anything.
However, once in every N copies, the new gene DOES do something - and a random mutation containing new genetic information has been created.
Does it make sense to think that DNA could randomly(without the direction of existing DNA instruction) get longer or shorter in length
Yes, and it happens all the time, even in human DNA.
, and still function?
Yes, for two reasons:
1) Most DNA is junk DNA. It doesn't do anything.
2) Once the error occurs, there are at least two mechanism that try to fix it. The first is proofreading, a process where the new strand is compared to the old strand, and if the incorrect amino acid has been paired with the template, it is replaced with the correct one. The second is mismatch repair, where deformed strands (i.e. ones with incorrect or too many amino acids in the new strand) are repaired. These mechanisms are themselves, of course, imperfect as well - which is why the genome sometimes grows and sometimes shrinks.
Do you believe DNA is really that unimportant?
It is neither unimportant nor fragile. 99.99% of the time it can still operate with replication errors - which is very good news for us.