Also, even at the lastest versions of pandas if the column is object type you would have to convert into float first, something like:
df['column_name'].astype(np.float).astype("Int32")
NB: You have to go through numpy float first and then to nullable Int32, for some reason.
The size of the int if it's 32 or 64 depends on your variable, be aware you may loose some precision if your numbers are to big for the format.