Hola Winry

What this means is "I am impressed" - you need the past participle form "maravillado", not the gerund/present participle. We are using an adjective here with estar that sometimes takes the same form as a past participle, which is this case. 

Other examples are:

Estoy obsesionado - I am obsessed

Estamos agradecidos - we are grateful

Está destruido - it's destroyed

Here is a lesson where you can see lots of examples.

I hope it helps.

Saludos

Inma