There are two πόσο. The first one is the interrogative pronoun "how many" and as a pronoun applies to nouns. This one has genders. It's πόσος-πόση-πόσο.
For example: Πόση σοκολάτα θες;=how much chocolate do you want? Or πόσα λουλούδια έχεις=How many flowers have you got? Or πόσο αλεύρι χρειάζεσαι=How much flour do you need? This version of πόσος is gendered and conjugated normally.
The other version is the adverb πόσο=How (much). This one applies to verbs and is the version we have in the above sentence. Πόσο όμορφος είναι!=how beautiful he is! (Here how applies to is) ποσο μικρό είναι το λουλόυδι;=How (small the flower) is? This version as an adverb is not conjugated and is non gendered (because verbs are not gendered ;) )
In English, an adverb can modify a verb, another adverb, or an adjective, and an adjective modifies a noun or pronoun. Are the roles of adverbs and adjectives like this in Greek? In this case in the English sentence, I think "how" is an adverb that modifies the adjective "small" not the verb "is" or "are". In the Greek sentence can πόσο as an adverb modify the adjective μικρό?