1. Ramban's source is Pesachim 94a. That same Gemara rejects that figure based on the Beraisa of Chagigah 13a that you quoted.

Note that it also presents the Beraisa which contradicts this value, giving a size for the world of 3600 x 400 x 4 Mil. Considering that the average person does 40 Mil/day (ibid.) and can walk approximately 300 days/year (without Shabbos and Yom Tov), this figure is pretty consistent with the 500 year walk of Chagigah 12a and 13a.

2. Tosfos ibid DH Kol points out that the other two Midrashim (the 500 years and one "Yishuv") about the size of the Earth are contradictory (even putting aside the 24,000 Mil figure). However, the figure Rashi offers there (DH Tachas) seems to reconcile the two Midrashim.

3. Regarding the scientific figure, if you look at the Gemara in Pesachim you will find it is discussing just a single hemisphere. (The hemisphere opposite Israel was not considered a habitable or inhabited region - see Rosh Hashanah 20b and Ba'al ha'Me'or). If so, the figure of 24,000 Mil is pretty compatible with the scientific figure.

4. As for answering the Gemara's (unanswered) question from the larger 500 year figure, apparently it includes much more than the Earth itself (about 31 million miles). Perhaps it includes the entire "Goldilocks zone", as it is called in which the distance from the sun is neither too little nor too much to support life.

Of course, this is what I would say to explain the Gemara about 500 years in its literal sense - the way Tosfos understands it and the way the Gemara seems to want us to understand it in the Sugya in Pesachim. However, there certainly are much deeper lessons cloaked in the Agadah. For instance, it might have to do not with physical distance, but with philosophical "distance" measered in terms of years of contemplation until it is fully understood. Nevuchadnezar the Rasha thought he knew everything and the Gemara (Chagigah 13a) is reprimanding him that he knew nothing.

This would of course vindicate Rava's 24,000 Mil figure of physical distance, in Pesachim.

Best wishes,

Mordecai Kornfeld

Kollel Iyun Hadaf