Questions on connecting and using the RS485 MAX14783E transirver
Pull-ups on the A/B lines mostly do not depend on the bus length, they are there to provide a proper logic level for your RS-485 transceiver when there is no active driver on the bus, in case the bus is open, and in the short circuit event.

Most driver in the past had a differential threshold voltage level from \$-200mV\$ to \$200mV\$, so that when you have no active driver on the bus, your RS-485 receivers are in the indeterminate state. To prevent that you want \$U_{AB}\$ to be bigger than the positive threshold value, so in case of \$+200mV\$ to calculate pull-up/downs resistors value \$R_{pud}\$ on A/B you simply use resistor divider formula:


$$ V_{AB} = V_{A} - V_{B} = V_{cc} * \left( \frac{R_{eq}+R_{pud}}{R_{eq}+2R_{pud}} - \frac{R_{pud}}{R_{eq}+2R_{pud}} \right) = V_{cc} * \left( \frac{R_{eq}}{R_{eq}+2R_{pud}} \right) $$

For \$V_{cc} = 5V\$, and \$R_{eq} = 54\$ (which assumes properly terminated line on both ends + some load) we have: 

$$ V_{AB} = 5 * \left( \frac{54}{54+2R_{pud}} \right) $$

Resulting into pull-up/down value \$R_{pud}=648\$ ohms. To give some margin, you want the value to be less than that, say \$R_{pud}=560\$, but not too low, otherwise you will have too high bus short-circuit current (which is very possible in RS-485 network), so it is always a trade off. The lower is your positive threshold value, the higher you can go with your pull-up/downs, if instead of \$+200mV\$ it is \$+10mV\$, you will get \$R=13473\$ ohms, which yields to the \$12k\$ that is commonly used, and so on. For MAX14783E you have a threshold voltage of \$-10mV\$ so in the most conditions you will be fine without additional pull-up/downs on the line. But considering the noise on the bus and that the input hysteresis is \$10mV\$ (and only typical value is given), adding external pull-up/downs can improve the robustness of your design.


It usually never hurts to add a pull-up to the UART Tx/Rx pins as the idle state of UART signals by default is logic one. Imagine you have no pull-up/down on your Rx pin and it's disconnected from the other device, now it is susceptible to any noise which might result in a state when you constantly receive garbage.

The same applies to DE/RE pins, if for instance you have a logic high on the DE pin, but your micro is not yet configured, then it will start driving the bus and might collapse a communication for certain amount of time (or even forever if it stucks in some erratic state).

In my RS-485 designs, I do add an external pull-up to MCU Rx (RS-485 transceiver RO pin), and a pull-down to DE/RE pin which I tie together most of the time.
The allowable common mode voltage for RS485 is indeed \$-7V\$ and \$+12V\$. So that on each line, A and B, you can have a signal within a range of \$0-5V\$ referenced to the transceiver ground, on top of that transceiver should be capable to withstand \$\pm7V\$ common mode voltage as a minimum to qualify as the RS-485 transceiver. Many transceivers on the market can provide much higher common mode noise withstanding ratings.