You know, we are pretty lucky. It could've been a lot worse. If the Germanic tribes that took over western Europe after the decline of western Rome hadn't been reconquered by the Byzantines and/or gradually converted to Trinitarian Christianity, then it's entirely possible Arianism would be a genuine denomination/tradition of Christianity today, as sizable or more sizable than Trinitarianism.
Arian Christianity would be the Germanic form of Christianity, dominating much of Western Europe, it's possible Italy could be an exception if the majority Trinitarian population didn't gradually convert to the Arianism of the upper class, yet having the Bishop of Rome there might be a significant factor in halting that unless of course he is somehow removed or usurped by later tyrannical Arian rulers. If the Western Europeans decide to colonize the Americas then Arianism could become the dominate form of Christianity in the new world, especially under Visigothic Spanish. Assuming Arian missionaries were sent to the Anglo-Saxons (which likely would have happened at some point) the Great Britian would be Arian and their influence would be felt in the Americas as well.
Catholic-Orthodoxy would be the Roman form of Christianity, dominate in the Middle East and Southern Eastern Europe and possibly in parts of Italy. Of course Islamic conquest would be a significant factor here and it's possible Arian Christians in the West and the Islamic conquerors of the East would both attempt their own conquest of the Byzantines, though not necessarily being on the same side. Spain would probably still be invaded and there would probably still be some kind of reconquista but it may take slightly longer.
Eastern Europe could be a battle ground where both Arianism and Catholic-Orthodoxy meet with rival clergy serving different groups of Christians as it was under the Ostrogoths when they were pushed out by the Hun invaders and set up in Italy. Or perhaps it would be one or the other, maybe the Arians have more influence and it becomes Arian with the rest of Europe, or maybe the Catholic-Orthodox have the most influence there somewhat similar to our timeline.
Early Arian rulers were more tolerant of Jews and it's possible Judaism might flourish more in Western Europe than it does in our timeline. Catholic-Orthodox Christians could face severe persecutions as they did under the Vandals in North Africa and Sicily, though the Ostrogoths were generally tolerant of Trinitarians.
Just remember folks, thank the Byzantines (particularly Justinian I) and the conversion of Reccared I from Arianism to Trinitarianism for the timeline we are in now, because it could've just as easily gone the other way, and billions of souls lost. Thank God that all forms of Christianity, even the darn Baptists, are Christians; non-Trinitarian sects are insignificant today. Thank God!