If you are new with virtualization, your clients does not understand that requisites for a virtual server differs from physical servers or even if you don’t have a baseline yet to measure your needs, there’s a website that you can use to check what the others are doing with their own VMs.
CompareMyVM allows you ,for free, on learning how to right size your VM, compare VMs by applications and other categories.
It could be a good start.
But my advice is always monitor and measure up the performance of your VMs, if you can overestimate your VM’s requisites and never revisit it you will probably be wasting a lot of resources.
Try to start small, monitor and increase as required…
It’s always easier to increase capacity then decreasing it.