groß
big / large
Describes above-average physical extent, height, or area.
Der Elefant ist ein sehr großes Tier.
The elephant is a very large animal.

big / great
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big / large
Describes above-average physical extent, height, or area.
Der Elefant ist ein sehr großes Tier.
The elephant is a very large animal.

As an adjective, 'groß' (big) is written with a lowercase letter. But when it is used as a noun (often after 'etwas' (something), 'nichts' (nothing), 'alles' (everything)), it is capitalized. This rule is important for correct spelling.
Das ist ein wirklich großes Haus.
That is a really big house.
'Groß' (big) is a basic word for many other German words. If you recognize 'groß', you can often understand the meaning of words like 'die Größe' (size), 'vergrößern' (to enlarge), or 'Großeltern' (grandparents) more easily.
Können Sie das Bild bitte etwas vergrößern?
Could you please enlarge the picture a little?
The literal meaning 'groß' (big) refers to the physical size of objects or people. In figurative meanings, it describes importance, intensity, or age, which becomes clear from the context.
Ein großer Baum spendet viel Schatten.
A big tree provides a lot of shade.
To express importance or quality, 'groß' (great) is often combined with abstract nouns. Typical phrases are 'ein großer Erfolg' (a great success), 'eine große Ehre' (a great honor), or 'ein großer Unterschied' (a big difference).
Seine Erfindung hat einen großen Unterschied gemacht.
His invention made a big difference.
To describe the intensity of a feeling or state, 'groß' (great/big) is used frequently. Typical combinations are 'großen Hunger haben' (to be very hungry), 'in großer Eile sein' (to be in a great hurry), or 'große Freude empfinden' (to feel great joy).
Nach der langen Wanderung hatte ich großen Hunger.
After the long hike, I was very hungry.
In the context of family and age, 'groß' (older) is used to distinguish older siblings from younger ones. It then stands directly before the noun.
Meine große Schwester studiert in Berlin.
My older sister is studying in Berlin.
The idiom 'im Großen und Ganzen' (by and large) is a fixed unit. It often stands at the beginning of a sentence and means 'considered as a whole'.
Im Großen und Ganzen bin ich mit dem Ergebnis zufrieden.
By and large, I am satisfied with the result.
long
'lang' and 'groß' both describe a kind of size, but they do not mean the same thing. 'lang' is used for extension in one direction, for example with a path, a table, or also a duration of time. 'groß' describes more the overall size, height, or area of something. Learners easily confuse the words because in other languages a similar word is often used for both. In German, you say: 'ein langer Film' (a long movie), 'ein langer Weg' (a long way), but 'ein großes Haus' (a big house) or 'ein großer Mensch' (a tall person).

Herr Meier sucht für seine Familie eine größere Wohnung.
Mr. Meier is looking for a bigger apartment for his family.
Der Weg zum See ist lang, deshalb nehmen wir die Fahrräder.
The way to the lake is long, so we are taking the bikes.