As far as the parameters of combat and the like. It is worth noting that Rambam expressly notes about when fighting an 'Ir' - city, the inhabitants are given the means to escape:
כְּשֶׁצָּרִין עַל עִיר לְתָפְשָׂהּ. אֵין מַקִּיפִין אוֹתָהּ מֵאַרְבַּע רוּחוֹתֶיהָ אֶלָּא מִשָּׁלֹשׁ רוּחוֹתֶיהָ. וּמַנִּיחִין מָקוֹם לַבּוֹרֵחַ וּלְכָל מִי שֶׁיִּרְצֶה לְהִמָּלֵט עַל נַפְשׁוֹ. שֶׁנֶּאֱמַר (במדבר לא, ז) "וַיִּצְבְּאוּ עַל מִדְיָן כַּאֲשֶׁר צִוָּה ה' אֶת משֶׁה". מִפִּי הַשְּׁמוּעָה לָמְדוּ שֶׁבְּכָךְ צִוָּהוּ:
When we besiege a city which we want to capture, we do not encircle it from all four sides, but only on three. We leave one side open for them to flee. Anyone who wishes to escape with his life may so do, as it says, “and you shall deploy against Midian, as G-d had commanded Moses” (Numbers 31:7). By Tradition we have learned that this is what was meant. (Mishneh Torah, Melachim Umilchamos 6:7) - (Also refer to the Sifrei Bamidbar 157)
Note it is specifically an עיר. When relating about an Ir Hanidachas - a city that has been earmarked for destruction due the city-wide idol worship, Rambam tells us in Mishneh Torah, Avodas Kochavim 4:2 that it is only an עיר that qualifies and relating to your first question he also writes a clear sizing definition:
...שֶׁנֶּאֱמַר (דברים יג טז) "ישְׁבֵי הָעִיר" לֹא כְּפָר קָטָן וְלֹא כְּרַךְ גָּדוֹל. וְכָל פָּחוֹת מִמֵּאָה כְּפָר קָטָן. וְרֻבּוֹ שֶׁל שֵׁבֶט כְּרַךְ גָּדוֹל...
...for it is said: "the inhabitants of their city" (Ibid.) which means neither a village (כְּפָר) nor a large city (כְּרַךְ), and every place of less than one hundred citizens is a village and of a majority of a tribe is a large city....