Translation:The Rhine is a large river in Germany.
You should checkout the comparsions in chapter's tips and notes in the tree.
excerpt from it: groß(normal), größer(comparative), am größten(superlative)
Now to clear your confusion.
größer is used to compare two things. eg: Ein Fluss ist größer als ein Bach.(A river is bigger than a creek).
Now groß can be used as an adjective with nouns. And adjectives get inflected based on factors like gender and number.
eg: großer Mann = big man, der große Mann = The big man, ein großer Mann = A big man
große Frau = big woman, die große Frau = The big woman, eine große Frau = A big woman
großes Auto = big car, das große Auto = The big car, ein großes Auto = A big car
große Jungen = big boys, die großen Jungen = the big boys
You can notice that the inflected groß in "großer Mann" looks like the comparative word "größer", but they are not the same word. They even sound different. You need to notice and remember the umlauts for this exact reason. There are words in german that look the same if you take off the two dots, but you shouldn't.
https://en.wikipedia.org/wiki/German_declension#Adjectives This wiki item will help.
im = in + dem = in the. There is only one Germany. When you use the determiner (das), you are apparently indicating that there is more than one Germany and that you are speaking about a particular Germany. So for unique things like a country name (Germany, India), God or other similar things, you don't use it's determiner when speaking about it, because there is only one in existence.
Der Rhein ist ein großer Fluss in Deutschland (no determiner because only one Germany).
Der Rhein ist ein großer Fluss in der Umgebung (determiner because there are virtually many surroundings/environments and you are talking about a specific surrounding).
Further notes: So "in Germany", "in India", because we know for sure their are only one of those. But "in the United states of America". Because their might be other states of the north/south American continents that are united that we don't know about.
Hope this helped.
And this is German for you. A select number of countries are used with an article, which then has to be treated as any (declension, contractions…).
Most, including Germany, don't, so it is indeed not *in dem Deutschland", but simply "in Deutschland".
Note there is also only one Rhine, and yet…