Novenas are always nine days, since the word’s very meaning is “nine days”. It originated from Acts 1, as Mary and the 120 disciples waited in the Upper Room in Jerusalem to pray for the coming of the Holy Spirit as Jesus commanded them. From Ascension to Pentecost was exactly nine days.
The Holy Spirit novena therefore was the first novena of the Church. Afterwards, many other devotion took after the format: nine days, not too long and not too short. And typically, such novenas were prayed in community, so there is a need to have common text so everybody can pray the same words together.
As to the other lengths of “novena”, I am not aware of the 3-day and 8-day ones. There are the more popular the 33-day, 40-day, and the perpetual novenas.
The 33-day format was best known with the 33-day preparation for Marian consecration taught by St. Louis Marie Grignon de Montfort. I believe he chose 33 days since Mary enjoyed 33 years of formation from Jesus, so symbolically we desired to be formed by Mary with the same fervour over 33 days.
The perpetual devotion is particularly popular with the devotion to St. Anthony and Our Lady of Perpetual Help. St. Anthony is usually on every Tuesday and Perpetual Help every Wednesday.
As for novenas to recommend, I would say that first you should get to know the saints. Once you get to know one, if his or her life speaks to you, you might be drawn to practice a deeper devotion by offering a novena through the saint.
Each saint also has his or her particular area of patronage. So if you have a particular need, get to know the patron saints associated with it, and you can pick one to offer a novena.
And where to find the novena prayer? Chances are you will find them on the internet easily. And if you can’t find one, you can just talk to the saint and offer some simple prayers, or you can certainly feel free to write one for yourself.