Have you read the advice on the RAC website?
The AA probably offers similar clarification.
As far as cars over 20 years old are concerned the advice is clear:
“although if you have a particularly old car that does not conform to any emissions standards, you won't be eligible for a vignette and you won't be permitted to drive your vehicle during those times when restrictions are imposed“.
Just make sure you avoid the restricted areas.
As to the file size limitation, reduce the resolution when you scan your V5.