Without invoking ASB's, whats the maximum plausible area that could be realistically Islamized. POD can be any time after 622.
Probably the best possible single POD is the Arabs taking Constantinople early on. It's been discussed a few times before on this board. The Byzantine Empire is finished, and Europe loses the bottleneck and buffer defense against Islam the Byzantines provided them. It's widely thought the Umayyads were planing on making Constantinople their capital if they took it, because it really is one of the most valuable cities in history. With Constantinople becoming the core of the Caliphate Muslims can project power pretty deep into Europe, and there's really nobody else in Europe who could stand up to the Caliphate at this time. It's likely that Muslims expand into the rest of the Balkans and over the Adriatic into Italy. A lot of central and eastern Europe was still pagan at this time and some of those groups which were Christian were only superficially so. So with Muslims in Constantiniple you can have groups like the Bulgars and Magyars converting. In this situation the Khazars will likely convert to Islam rather than Judaism, so Muslim influence will spread further north into the steppe and OTL Russian area. The Byzantines were the big cultural/religious influence on the Rus so in this situation the Rus likely become Muslim rather than Christian. And in OTL the caucasus was surrounded by Christian, Muslim, and Jewish powers, and were subject to competing influences and could play the three powers off against each other. If the Byzantines are conquered and the Khazars convert to Islam then the Caucasus will be surrounded by Muslims on all sides so the Caucasus will be much more Islamified than OTL.
The Arabs taking Constantinople would be hard, but if it could be done the consequences would be immense, it would likely lead to Muslim Balkans, Muslim Italy, Muslim Caucasus, and Muslim Russia. If the Muslims can win at both Constantinople and Tours then they'll take even more of Europe, and Christendom will likely soon be reduced to Germany and Britain only. Then, when the colonial era starts eventually you can get Muslim Americas and even more Muslim Africa.
Weren't there still a lot of pagans in Germany at this time? In a world with Islamic Constantinople and Islamic France, they would probably come under increasing pressure to convert. As for Britian, I can imagine them being taken by an Islamic equivalent to William the Conquerer in the 10th or 11th century...by the 13th century Christianity could be a minority religion in almost all of Europe.
Does any one have ideas for the east? Say, a Muslim-majority India, or a greater Islamic presence in China?