?
Might get in on this, but there are a number of sleeves that fit Poke cards.
The ‘looser’ fitting ones I have had experience with are the Ultra Pro Card Sleeves, 2-5/8" x 3-5/8" (or 92mm x 66mm).
For double sleeving, I have used the KMC ‘Perfect’ Sleeves, which are 89mm x 64mm. This sleeve will comfortably go inside the Ultra Pro as a double sleeve, or either as a single.
There are a heap of others around though, HTH
you could just buy 3 - 4 different ones, and see how many you can cram onto one card. 
i think usually the max is around 3.
but should start at the medium card sleeves generally, anything 89mm x 64mm upwards is good.