As for the synagogues, the first Christians did indeed meet in synagogues on the Sabbath at the very beginning, which largely consisted of prayers and hymns (mostly psalms and other scripture), and particular scripture readings. They would then, either that evening or early the next morning, on the first day of the week (or the eighth day) meet for those elements which were specifically Christian. Not too long later, they had to stop attending the synagogues and they tacked those synagogue elements of their worship directly before their christian elements of worship, which is what created the pattern of the liturgies/masses we see today: the first half being prayers and hymns culminating in the reading of Scripture, and the second half being prayers and hymns culminating in the reception of the Eucharist.
As for the rest of your question, there are early house churches that have been unearthed (I don't know if any of them are first century or not), and while we don't have attendance records, if we assume that the number of people that can fit in a large house was the number of people attending each house church, and perhaps a little more, then we can probably assume that they had around 50-75 people in attendance at a given house church, or somewhere thereabouts.