Here are several similar blocks in MSL: CombiTable1Ds, CombiTable2Ds, and CombiTimeTable. Not sure which you mean and I use CombiTimeTable, but the same principle described below holds for all of them.
You need to provide the table size information to CombiTimeTable at time of translation. Thus at least a dummy table that settle the size and you can read that from a file.
After translation and before simulation, you can change the values in the table but you cannot change the size.
The details of how to make your file in a proper way you find here:
I am not sure I understand correctly what you means with "I want to dynamically assign it", but it sounds to me like after translation and before simulation, and then you have the limitations as described.
I must also admit that I have not access to Dymola and I base my answer on what holds for Modelica in general.