The bulb measures 1-1/3" diameter according to Kayla's review. While that is not huge for anyone with any anal experience, it will probably be quite large for your partner if he has never done any anal play before. You may want to experiment with fingers first to see if he is okay with anal play. There have been many threads discussing how to start anal/prostate play in men if you search the forums.
Also consider that with the Feeldoe, he will be penetrating you vaginally with his penis while the Feeldoe is in your anus in the missionary position but reversed if you are doggie. The opposite will generally be true with a harness because the dildo is usually positioned above his penis, such as with the Deuce.