Linux Image Size Limitations at 30GB

Hi-

I'm new to Azure and I'm trying to replicate a hosted server with a large system disk.

I'd like to make my linux system disk image 300GB instead of the standard 30GB when I provision it.  How do I go about doing this and is it possible/recommended?

It is not preferable because a large system drive would affect IOPS performance?  

Would attaching additional disks give me much better performance? 

Do the provisioned system disks have less performance that the attached disks?  If yes, why.

Thanks!