My partner and I are too much interested in anal sex. but we are facing problem that while inserting penis in her anal she gets too much pain and consequently we remains away from satisfaction. Is there any surgery to increase size of anal or any sex toys available to increase size?

Anal sex is always painful. Its tightness is natural and required for proper defecation (passing stool). There is no surgery available for this and even not desirable also. You may try lubricant like KY jelly. If you follow my advise, you should avoid doing it.