You are asking simple questions that end up with complicated answers. SQL Server is best treated in a holistic manner. As an example: disk IOPS won't matter if you don't have sufficient RAM... paging will occur and you lose your disk I/O benefits.
As for sizing a database... create it in DEV and know your data. Estimate the retention periods of data and the change deltas that will occur. Then double it.
Then baseline your PROD dB and monitor its growth.
The bottom line is there is no magic formula that covers every situation. A few indexes added to a dB can throw out a lot of your previous assumptions. A new data retention policy can soon outgrow the hardware. 50 new users can be a tipping point for issues to arise.
It also depends on the environment you work within. Are a lot of instances consolidated or is this a dedicated server? Are you using a SAN and able to grow as required? Are you running as a VM?
Some smarter people than me will be able to give you some guidelines and formulas that can help but the important points for me are that you must have intimate knowledge of your data and the holistic nature of SQL Server.
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