Perfmon Counters that relate to SQLIO
SQLIO is technically deprecated - it's been replaced by the newer utility Diskspd. However, the same basic answer applies.

If you look at an existing server and guess how much storage throughput it's USING, then look at the counters:


IOPs - Physical Disk Reads/Sec and Writes/Sec
IO size - Physical Disk Read Bytes/sec divided by Reads/sec (and same for writes)
Latency - Physical Disk Avg Sec/Read and Sec/Write


However, this only reflects what your current storage is able to deliver, not what your server would rather have. Think about it as drinking water through a small straw. If I ask how fast you want to drink, and I measure your consumption through that tiny straw, it doesn't tell me how fast you could actually drink if you weren't constrained by such a small straw. Instead, I need to hook you up to a pipe (or straw) with unlimited throughput, and then measure how much you actually consume - and then use that to size the storage you need. (Or want, anyway.)