From Import-Csv the value is a [string], you could check by veryfing the type of this Parameter.DiskSize
Why don't you use 2 seperate columns in the CSV and use 2 variables on the DiskGB parameter.
The CSV
....,Disk1,Disk2,....
...,1,1,....
...,2,1....
When you import the CSV file you will get 2 variables $_.Disk1 and $_.Disk2.
Now feed those to the parameter
New-VM ....-DiskGB $_.Disk1,$_.Disk2