There are two important variables and one important caution here. The caution is that containing grasshoppers in an area may be very difficult, You may have to have insect screens fully covering an area. Keeping flying migratory livestock contained is somewhat difficult.
The variables regard climate. Grasshoppers don't survive freezing so you have a limited growing season, and the shorter the growing season the more land you will need.
Third is water. The more lush your landscape the less of it you will need.
So assuming you have ideal areas and can sustain a population of grasshoppers at 30/m^2 (this may be optimistic), with each grasshopper weighing about 0.1g, then each M^2 would produce about 3g of meat/year. 1kg = 1000g so it would take approx 333m^2, and so your 100kg would require about 33,333m^2. I don't know what you could grow that would sustain a population that size for very long (bamboo, maybe?) so you might want to assume at least 3-4x that size for sustainability reasons. I would therefore estimate approx one hectare to be safe, more if land is marginal.