Lang is long and langsam means slow? I wonder what the connection is between these two words.
Just think, Langsam, slow, taking a LONG time; Lang, Slow. So, we can deduce from that that Langsam has the root Lang, and the ending sam, to mean time, therefore, Long time.
Actually, -sam does not mean 'time'. It is a suffix used to form adjectives and adverbs, but it does not usually add much to the meaning or considerably change it (some more examples: einsam 'lonely', gleichsam 'in similar manner', bedeutsam 'significant').
-sam is a cognate of the English suffix '-some', as found in 'tiresome', 'loathsome', 'fearsome'. In fact, forms of this suffix are found in a number of Germanic languages.
Alternatively "Der Fisch ist groß" should also be possible as translation since a long fish would be only one, that has its length as the most striking/prominent attribute.
Usually, we (I'm a native speaker trying this course as reverse English learning opportunity) use the expression "groß", so the more common expression would be "Der Fisch ist groß." (The fish is big.)
But it's not inherently wrong this way, just very specific to the situation shown on the image posted by mucca12 ;-), for example
It depends on the object or person(s) they refer to: "Ich habe lange Haare." (I have long hair) "Das dauert lang(e)." (That is taking a long time./That takes long.) "Die lange Limousine." (The long limousine) "Über kurz oder lang müssen wir etwas ändern." (Sooner or later wie need to change something. -> Idiom "Über kurz oder lang, ...")
But there are more endings to "lang" possible, too, like "langer" (example: "Vor langer Zeit, ..." -> Long time ago, ...), "langes" (example: "Das ist ein langes Stück Holz." -> That's a long/large piece of wood.), "langen" (example: "Die langen Bretter." -> The long shelves/boards/planks.)
It's the cases and objects that basically define which ending is needed. Maybe learn some theory about these and you'll get the hang of it more easily. Grammar Theory isn't really taught here in this context, as far as I've found so far, though.