I'm assuming dogs (psi) are Masculine animate. Then why is it velke and not velci?
Because it is acusativ. That is also why PSI (nominativ) change to PSY (acusativ).
I writed "Velké psy hledám" as answer and It says It's a wrong answer.
If anyone is wonder why it isn't "velke pes hledam" that is because pes is one dog but psy is plural. The order doesn't exactly matter but your words need to be correct grammatically. Make sure to do so.
You also need the accusstive case singular "velkého psa", plural "velké psy" instead of nominative "velcí psi".