I've never done an install of Laserfiche in Azure. The following is just my observations based on how Azure storage offerings work.

Initially, you should try to match your IOPs to what you currently have, and then scale from there. You should be able to use the tools on your SAN, or the Windows profiling tools to figure out what you need.

Also, you don't have to keep everything in the premium tier, since it can get expensive. You can have different volumes on different types of storage, depending on how often it gets accessed. There can be a 3x price difference between HDDs and premium SSDs. You can always start with standard SSDs and the move some volumes to something faster later on.

Storage on Azure is a lot different than local storage. By default, all storage is locally redundant. So, if you ask for 1 TB of storage, Azure will transparently manage two additional replicas of your data. There will always be two copies of the data in one rack, and another in a second rack. I only mention that, because it means we need a slightly different look at how storage is provisioned in the cloud environment. It's all done in software, and you're not actually connecting directly to physical disk. So, size and performance considerations get a little fuzzy.

For the slightly different option, you can look at something like Azure Files, which operates like a file share served from a SAN. The pricing is different, and you don't need to provision a set size ahead of time.