Welcome to the Bunnings Workshop community @rachel1904. It's fantastic that you've joined us and many thanks for your question.
If you are happy with your current blinds, then measuring them would be a great place to start. If you are after a non-standard sized blind to fit perfecting within a recess, you might like to consider our custom made selection. Here's Riva's guide on how to measure Venetian blinds and our online catalogue.