"High extent" is a commonly used phrase that denotes a large degree or amount of something. However, there are numerous synonyms that can be used in its place, including "large proportion," "significant magnitude," "vast scope," "substantial level," "considerable range," "extensive magnitude," and "great measure." Depending on the context in which it is used, other appropriate synonyms might include "massive scale," "pronounced intensity," "notable depth," and "far-reaching reach." Using alternative synonyms can help to add variety and nuance to one's writing or speech, making it more engaging and effective.