After some discussion with Campbell, changed the way cstruct sizeof is fetched.
Moved DataSize() to Blender.Types.CSizeof(Blendertype). Supported types return sizeof(data struct), otherwise -1.
To quickly check what types are supported:
import Blender.Types as bt
x = dir(bt)
for t in x:
if t[0] != '_':
s = 'bt.CSizeof(bt.' + t + ')'
print t,"=", eval(s)
47345cfd78