None of the Rishonim state explicitly that a pirtzos esser is a d’Oraysa – Rav Aharon Kotler zt"l and others extrapolate from some Rishonim as such. However, the fact is three Rishonim state unequivocally that pirtzos esser is only d’rabbanan (Hashlama, Eruvin 5a; Tosafos HaRosh, Eruvin 17b, and HaEshkol, p. 167). Furthermore, even those few Achronim who state that a pirtzos esser is d’Oraysa would admit that when utilizing a tzuras hapesach to close the pirtzah it is as though the pirtzah itself ceases to exist (Mishkenos Yaakov, O.C. 122 p. 144). Additionally, it’s difficult to understand Rav Aharon’s (Mishnas Rav Aharon, 6:2) statement that Rabeinu Chananel (101a) maintains pirtzos esser is d'Oraysa. Since Rabeinu Chananel is referring to Yerushalayim and Rav Aharon adds there was shem daled mechitzos there, consequently, according to R’ Yehudah, the pirtzah would have to be at the minimum 13 1/3 amos. Why would Rabeinu Chananel only go according to R’ Meir (even more so, the Yerushalmi states this part of the Mishna is according to R’ Yehudah). Moreover, according to the way the Meiri (11b) explains the Rabeinu Chananel we see that he maintains pirtzos esser is d’rabbanan. (See Divrei Yechezkel, siman 5:13 for a different explanation of Rabeinu Chananel, 101a.)
When clarifying that a pirtzah of ten amos is a rabbinical proscription, almost all poskim do not differentiate between a pirtzah of ten amos and one of thirteen amos which is proof that they maintain there is no difference halachically between them. Additionally, there are poskim who clearly state that a pirtzah of thirteen amos is me’d’rabbanan as well (Zera Emes, Eruvin 17; Bais Ephraim, O.C. 26; Avnei Nezer, O.C. 265:25, 276:1; Aruch HaShulchan, O.C. 362:26, and Chazon Ish, O.C. 107:5-8). Furthermore, even those few poskim who state a pirtzah of thirteen amos is me’d’Oraysa would admit that when utilizing a tzuras hapesach to close the pirtzah it is as though the pirtzah itself ceases to exist (Mishkenos Yaakov, O.C. 122 p. 144).
Regarding a pirtzah of sixteen amos even the Mishkenos Yaakov (O.C. 122 p. 144-45) admits to the Bais Ephraim that it is not a qualifier of a pirtzah in a mechitzah.