It’s hard to see, but the negation is actually ne … que meaning “only,” so literally it means, “I made this one longer only because …”

Je n’ ai fait celle-ci plus longue que parce que…

Actually, you will use «ne … que parce que» to answer a negative question. So your sentence must be the answer of a question (implicit or explicit) like :

Pourquoi ne l’avez vous pas faite plus longue ?

It may imply you are asked to justify about what you did. You could also say :

J’ai uniquement fait celle-ci plus longue parce que…

which means the same thing.

Adding some other sentences with the same structure :

Je ne fais que passer
I am only passing through

Il ne va au sport que pour se vider la tête
He only goes to sport in order to empty his mind

Les cloches de l’église ne sonnent que le jour
Church’s bells only ring during the daytime