My rule: Start with 1 vCPU, 1GB memory (or more if you're expecting a heavy load, less if you know what you're doing), you can always power off the box and increase both of these at a slide, going down has caused problems in Windows 2003, I hear it's better now though...
But this practice prevents you from overallocating VMs and never fixing them and having poor consolidation ratios, unused vCPUs means wasted cycles (hypervisor needs to schedule dead CPU time for the VM), overallocation in RAM just means eating more vRAM for OS cache and other waste (for the most part).
It completely depends on load though, what are you doing, how much of it, etc.
