What is the degree of sin when having sex via anal with your wife, is it greater, lesser sin or is it makrroh?
"On the day of resurrection, Allah will not look at a man who had intercourse with his wife in her anus"—Narrated by Ibn Abi Shayba, 3/529; narrated and classed as sahih by Sunan al-Tirmidhi, 1165
"Allah is not shy to tell you the truth: do not have intercourse with your wives in the anus."—Narrated by Ahmad, 5/213
The rest of the Hadith can be found here. "Islamic views in Anal sex"
Well, as far as I know, Prophet Muhammad (pbuh) did not give a complete list of sins in the increasing/decreasing order of degrees/magnitude. Only if he was asked by his companions he replied by ranking a few as many times his companions asked.
But, if seriously are curious in knowing up to what extent could Anal sex be Haram or considered sin, then it is directly related to, up to what extent are the Hadith which talk about it are authentic. Today, relative grading, we can't do it. But atleast, they'll not be in the top 5 per se, cuz they're some other sins which the Prophet (pbuh) himself graded.
So, in general, if something has been prohibited, its prohibited. That's it. Checking where does it stand in the list or what rank does it have, is, illogical and waste of time.
May the creator guide us all.