User blog comment:B214/Shin DxD 1 discussion/@comment-77.182.212.66-20180928234632/@comment-108.7.146.103-20180929003724

Issei currently in the LN is biologically a humanoid dragon reincarnated into a devil. He is made of the flesh of Great Red formed through Ophis' power. He can dragonify his limbs (and in the future EX timeline, his whole body). The first time we see this dragonification, it easily over-powers the defensive magic circles of 3 rogue mid-class magicians, and by this point in the story he's far exceeded that strength. Despite this he lacks any base-form feats on par with an Ultimate class devil, mostly because his magical power is still too low, unlike Vali, who is essentially a Super-Devil. It will likely take time and training to reach that level of strength

TL;DR His base form is a B-class according to Indra, and his base-form, feat-wise, is at the high-end of the mid-class spectrum. In other words, currently he is not Maou class in base-form, but in the future he may gain that power.