|xx = Ubound(cells())|
This will put the size of the array into xx.
If the size of the array is unknown at runtime you can use dynamic arrays.
If (xx Mod 5) = 0 Then
ReDim Preserve tekst(xx+5) As String
This will increase the array length of texst() in multiples of five or whatever other number is appropriate.
The ReDim Preseve is a useful function when using Arrays of undetermined length. I am not sure that it is available in VBA as it is taken from VB6.