«Ну́жно» is used with infinitives (ну́жно спать = it's neccessary to sleep) and nouns of the neutral gender (ну́жно яблоко = an apple is neccessary). «Ну́жен» is used with masculine nouns (ну́жен сок = juice is neccessary), and «нужна́» is used with feminine nouns (нужна́ вода = water is neccessary).
You can see the gender of the noun in a dictionary, or guess it by looking at the ending of the Nominative case form (masculine nouns usually end in a consonant or -ь, feminine nouns usually end in -а, -я, sometimes in -ь; neutral nouns end in -о, -е).
The structure of the Russian sentence is different from the English one. Russian is closer to 'To-him is-neccessary size larger'.
Adjectives (in fact, most modifiers except её/его/их) are put before the noun if they change their form to match the gender, number and case of that noun. Otherwise, they are put after noun.
«Побольше» 'bigger' is an indeclinable form of adjective. Indeclinable adjectives or forms of adjectives come after nouns.
See my comment here for examples: https://www.duolingo.com/comment/11877095