Hi, Yes, you can delete the virtual disks in the controller and create one large virtual disk. In the system setup make sure that it is set to UEFI boot and not BIOS boot mode. This is needed to boot to a virtual disk larger than 2TB. 
I have procured Dell R730 server with 1.2 TB x 4 HDD. It's shipped with default RAID 5 with two logical volumes, 1.8TB & 1.3 TB each. So when I install Windows 2008 R2 OS, I will have two partitions C with 1.9 TB and D = 1.6 TB Why is this raid defined in this way? Is it possible for me to wipe out this default raid and define a new raid5 as 1.4 TB x4 HDD as a single raid volume? 
Dell R730 RAID5 and C drive size for Windows OS