Please refer to this page about how to conjugate a verb. You will need to be able to conjugate verbs in both English and in French, including many different tenses, before you finish your study here. https://www.thoughtco.com/introduction-to-french-verbs-1371059
Both are possible, depending on context (ie any hint that the person you are talking to is a man or a woman?)
In addition, if you get "you are tall" it can also be a polite address (with agreement in feminine or masculine singular) or it can be plural and in that case, you can get: "vous êtes grands" or "vous êtes grandes".
The French talk very fast & words like es& et, est etc are kind of said in such a way that they are unheard by the English speaking. It's usually always said but is barely perceptible in some sentences, especially when the French talk their normal speed. Also, sometimes es is from être, to be, informal, singular. Est means is, so the context should give you a hint of which word is being said. If the next word is a vowel, in most cases s is said, but not all. But as in tu es un garçon , s is not heard bc those words are said quickly & not readily heard as separate words.
I disagree a bit with you dc1577. You mention ES, in the exemple above, we don't pronounce the S, it's not because we talk fast, but in that case, we don't pronounce it. But, if a v oyel or a h follow, of course, we make the _liaison, I mean the link between 2 words : je sui-z-un garçon.That has nothing to do with the speed.
A tip is to really learn these two, since the verb être (to be), of which es and est are two forms, is very common and you really need to know all forms of present tense for the future, and the verb manger (to eat) is a model verb, that is, all verbs ending with er in its infinitive form (only one exception:aller) follow the same conjugation pattern: the endings of these verbs are all the same: e, es, e, ons, ez, ent.
Give it time and you will learn. You will need to learn how to conjugate verbs, both in English and in French.
- Je suis = I am
- Tu es = you are (where "tu" is the singular and informal "you")
- Il/Elle est = He/She is
- Nous sommes = We are
- Vous êtes = you are (where "vous" may be either the singular/formal "you" or the plural "you" regardless of familiarity)
- Ils/Elles sont = They are