Hi Paul
Some answers - I hope
1. You signed up with mydomain.com - but it is still an account under your reseller package like any other you create. To increase it's space see 2
2. In WHM go to the packages section and create/edit packages - you can specify limits for disk space and bandwidth usage. Then once the packages are created to your satisfaction - go to the account functions and upgrade/downgrade an account and assign it to your chosen package. Note you can create as many different packages as you want
3. Namesevers - when you signed up for the reseller package you will have recieved an email giving the two nameservers you need to use - these are the same for all domains you host
4. Think private nameservers are only available on
VPS Hosting and above.
Regards
David