Coding a 2D JRPG?

What is the limit on number of fields, field event scripts, world map size, and field entrances on the world map?

For the world map, I think the only size is 128x128 (tiles). For the event script there is no real limit, the game just switch to overworld script from a regular event script, so the vanilla limit is limited by the script space in banks $CA-$CC but you can call event code anywhere in the ROM (above bank $CA). There is no proper event triggers on world maps, all world map event to my knowledge have a regular event prior to them. There are 2 kinds of entrance triggers, there is no real limit to their number beside the room for data you have in the ROM (relocating them is possible to have more space). I'm not 100% sure but I think only 1 kind is used on world maps, the single tile entrances. The kind that has a width and height is reserved for open spaces on regular maps, such as a town entrance.