Yes, you can.
To decrease the size of emoticons click on the ‘Brosix’ menu in the application and select the ‘Settings’ option. From the pop-up window select the ‘Plugins’ tab on the left and then select the ‘Text Chat’ option. With this option selected, click on the ‘Settings’ button on the right. From the ‘Chat Settings’ window deselect the ‘Show single emoticons larger’ option in order to reduce the emoticon size.