No, it’s not too big. The size of the RK3588 is 23x23mm (I checked on the Rockchip website), if you are planning on getting the passive heatsink, get one that is 25x25mm. But in my trials of the passive heatsink, RK3588 was getting to about 45-50C. I switched to this one, https://www.amazon.com/dp/B00O4UIZ86?ref=ppx_yo2ov_dt_b_product_details&th=1, and it is consistently running at 25-28C.
146mm x 102 mm is the size of the whole mini-ITX board, not the chip.
50 °C is perfectly usable and indeed quite cool for a CPU.
50C is light use, 85C is Youtubing. It’s still OK though but you misread that. 50C is only like basic usage, but once you do a bit of Youtube, a bit of 3D it can still go up to 85C BUT it is OK on those temps.
No doubt, 50C is perfectly fine but 27C is even better 
Yeah 50C is perfectly fine but 27C sounds unrealistic, like the device is off Still like I mentioned it did gets to 85C when doing YouTube so tiny heatsinks should help it a little.
This is true the moment you start doing larger than 1080p you do need to cool it down so there isnt the skipped frames. Using NsCDE you can easily see how loaded the CPU’s are trying to run 4k. TheCPU at 80 is fine for a short time for the CPU but it slowly affects everything else due to the ambient temperatures.