At the bottom of this data sheet: https://datasheet.lcsc.com/lcsc/1811151645_CoreChips-SL2-1A_C192893.pdf the reference circuit diagram shows huge 100uf decoupling capacitors connected to the USB plugs. My circuit board is tiny and I am hoping to use a 0402 or even 0603 part, can I get away with 10uf (0402), or say 47uf (0603) caps instead? The hub is for a very specific use, I will be connecting two wireless keyboard dongles to it (semi-permanently), it is not going to be used as a general purpose hub.
According to Table 7.7 of the USB 2.0 Specification any compliant USB hub must include at least 120uF of bypass capacitance to prevent droop when a device is plugged into one of the downstream ports. Note that this is "per hub" rather than per port.
All of the electrical requirements are found in section 7 of the USB 2.0 specification.
But in your case, you're not designing a compliant hub, so you don't need to worry about droop during attachment. In that case its certainly possible to use much less capacitance.
Yes, you can use much smaller capacitors - 10uF would be enough, or even 4u7, as long as it’s an actual capacitance that you measured at 5V.
Do measure the capacitance under DC bias. The physically small capacitors use dielectrics that cause monumental loss of capacitance as the voltage on the capacitor goes up from 0.
The “47uF” part you think you’re using at 0603 will probably be an 8uF capacitor at best at 5V. But measure it yourself, and ensure you are set up to do capacitance measurements under DC bias. Unbiased measurements of surface mount capacitors are mostly fiction unless you actually used them with no DC bias. You should instead measure them under an operating voltage present in the actual circuit.
Many cheap capacitance meters can’t deal with DC bias, so you’ll need a better meter I bet. In fact, meters not designed for DC bias can get destroyed if you attempt to bias the capacitor.