"My sister's suitcase is big."
Translation:La maleta de mi hermana es grande.
Because gran is placed in front of the noun, with a slightly different meaning.
<noun> grande means that the noun in question is big (relating to spacial dimensions). gran <noun> means the noun is great (relating to how noteworthy or fantastic it is).
So for instance a chair can be really big, when it's as high as a house. But a person might be really great if they've done some amazing deed - like build duolingo ;)