How much uplink capacity (in terms of GB per day) is typical for a satellite in LEO? What aspects determine how much data can be transferred back to the earth?
Worldview-4, a high-resolution imaging satellite that has just been launched in LEO, downlinks up to 18 Terabytes per day:
This is achieved through a network of ground stations all over the globe, and a high-speed X-band datalink (800 Mbps). Note Worldview-4 is extremely expensive, cutting-edge technology and most (if not all) LEO imaging satellites downlink lower volumes.
Current LEO communication satellites have lower throughputs. For instance, the current Iridium satellites have only 40Mbps of crosslink throughput (https://en.wikipedia.org/wiki/Iridium_satellite_constellation), so even if they are used at maximum capacity all day it only aggregates to 0.4 TB per day.
Edit: Typically maximum downlink volume is determined by: link budget (basically your communication hardware), energy budget (how much energy your solar panels give you and how much you allocate to the comms system), and comms passes over ground station (depends on your ground station network, you can use inter-satellite links instead). This has to match the required downlink volume, which is the data produced by your sensor (basically depends on the number of pixels of you sensor for an optical camera, and the data compression methods used) @pericynthion can probably give you real-world orders of magnitudes for cubesats.
The record is 134 Gbit/s. But that's a new geostationary communications satellite, i.e. the whole satellite does nothing but receive and transmit data.