We are official UK stockists of Sanik products. The Sanik 2/3 AAA 400mAh NiMh cells are 30mm long or 2 thirds AAA size. They fit the iDect X1 and iDect X1i. Please remember to charge them in the handset on the base unit for 16 to 20 hours before making calls.
A good comparison can be found here showing the original BT product.