I was annealing brass the other day and I sized and decapped the cases before starting. I was wondering if the annealing process can change the neck dimensions and should I resize after rather than before?

It won't change the dimension, but, it does change the tension. It's a good idea to run them through your neck die to make sure the tension is good.

Cheers.

While I can't answer the question, the reason I anneal my 404 Jeffery cases is to prevent case loss through neck and shoulder splitting or pin holing therefore I don't run the risk of this happening by another sizing and de-capping process prior to annealing. To be honest any case splitting I have had occur is on firing but just to be safe I manually knock out the primers using the sizing stem with a hammer while the case is held up through the top of the press.
Because I dunk the annealed cases in cold water to cool, having the primer removed aids the drying process. I then F/L resize my cases after annealing.

As long as you are monitoring the temperature of the anneal,no, no change to the case dimensions.

Anneal first; after all, you are trying to avoid working the hard brass so by sizing first, you have voided what you are trying to accomplish.
Not because it changes the size though.