His store is too small.
First of all, you need the verb "is" in your sentence. Secondly, the word for trop is "too." Too in this context means excessively, to a higher degree than is desirable, too much.
The "his" part of this sentence is "son." That is the possessive adjective.
One additional thing to remember: Unlike English, French possessive adjectives agree with the object and not the person possessing them.
Son chat = his or her cat
Sa lettre = his or her letter
Son histoire = his or her story
After a recent visit to an historical site in the UK I finally made the connection between English "shop" and American "store". I toured the tunnels at Fort Amherst in Kent, built initially following the Dutch invasion of the River Medway in 1667, and greatly extended in Napoleonic times against the threat of a French invasion, which never materialised. Something about Waterloo, which put a stop to his gallop.... However, revenons encore a nos moutons. The final point of the tour was a look at the powder magazine, where a couple of thousand barrels of gunpowder were stored (beneath a guardhouse full of smoking Redcoats, but I imagine people were no different then. Give me a rule and I'll break it.) Which brings me back to "shop". Magasin A place to store stuff until you use it, or until someone comes "shopping" for it. Store. A place to store stuff until it is sold. Magasine a place to store stuff until you want to make it go Boom! What about shop? Old French échoppe (a lean-to booth), German Schopf (a porch), Old English shippon (a cattle shed), Middle English "shop" (a booth or shed for trade or work). Back to a storage facility to keep stuff in until you can move it on.
Am annoyed as I said His SHOP is too small which I know is right! Am English, ( living in France) and we say shop rather than the American option 'store'. You need to give your system greater answer vocab, given u have many nationalities doing Duolingo! I like your app but this happens so often!