You have two options:
1. If you want simple to use varialble, which always will have this value, you must declare it as constant:

[in General Declarations]
Private Const Var = 10

[in some sub or function]
Dim VarName (0 To Var) As Double

2. If you want to declare array with variable number of elements (dynamic array), you must use ReDim operator:

Dim VarName() As Double
...
Var = 10
ReDim VarName(0 To Var) As Double