Can anyone help me find out what value C12 should be?
Your load resistor is 100 Ohms. C12, in series with the 100 ohms wants to have an Xc of a lot less than 100 ohms (less than 10 ohms is good) at the baud that you will be using. Many similar circuits replace the capacitor with a short-circuit. The purpose of the capacitor is to reduce power consumption when there is no data flowing. If it were me, I would use a 10uF (because I have a lot of them in my parts bin!)although many different values would probably work well.
Thanks! I'll try a 10uF then, using 100nF now.. There must be something else wrong with the whole circuit (this image is just a piece of it) RXD and TXD connects to a PCA82C250 and then to CAN bus, it communicates but seems to lose data...