You can easily limit the record count if readings from the same device are stored together for a given time slot. For example, one hour of readings account for 20 messages If frequency is 3 minutes. With a message length of 40 bytes you will have a record slightly over 800 bytes when you add IDs and timestamps.
So each device will have its 24 records a day or 1440 for two months of data and the whole db will contain144000 records for the 100 devices.
A number easily managed by any db.