应该是外面下很大雨。或者外面的雨下得很大。因为雨不能用多来形容。可以说倾盆大雨， 绵绵细雨等等。

你可以说：“外面的雨很大。” 这个是最简单的表达方式。

I don't have something more to say. I'm just thinking, I don't have heard "外面下很大的雨" quite often, maybe we just use "外面雨下得很大". ~~

we don't translate"a lot of rain outside" directly, we do the same traslation as" it rained a lot outside" or "it rained heavily outside" So it should be 外面下很大的雨~

I think u may say 外面的雨很大,when we describe snow ,wind and rain,we will use 大 not 多。雪很大，雨很大，风也很大。