What is the preferred relationship title for "my mother's mother's big brother's son"?
You can use '表舅', because he is your mother's '表哥'.

Basically I think there are three prefixes that you can add to relationship words: '亲' (directly/closely related, which is usually omitted), '堂', '表'. All your ancestors, siblings (that share at least father or mother with you), and descendant are '亲'. '堂' only refers to your father's brothers' children. In this way, your '堂' something should have the same family name with you (assuming all children using inherit their family names from their fathers). '表' is used for basically all other kins that have a common ancestor with you.