Conjuration, mechanically, works like any other spell (blocks, aspects, maneuvers, attacks - or the solve impossible problems.) depends on the flavour you're going for.
The brick wall would be a border equal to the shifts you put into the spell.
The Frogs would be a scene Aspect. If you want other mechanical (dis)advantages, you build them into the spell.
Frog spell:
- 4 shifts (raining frogs, scene aspect)
- 10 shifts for 5 zones - probably more to fill a whole park
- 3-shift (weapon 0), attack in all those zones (I remember cars taking damage from the falling frogs but that might just be fluff)
- 2 shift zone border ('cause squished frog guts are slippery)
Total shifts = 19
It depends how you want to design the spell. A scene aspect might be all you need...3 or 4 shifts. Then let players and NPC's tag/invoke the aspect for whatever creative stuff they can think up.