On the MX, a VLAN of 20 named "Uplink" is defined using 192.168.128.0/24 using an MX IP address of 192.168.128.254.
On the MS, a VLAN of 20 named "Uplink" is defined using 192.168.128.0/24 with an interface IP address of 192.168.128.1.
Does the size of the subnet matter for this uplink VLAN? The example uses /24, but should it be much smaller or is the size completely irrelevant. For example, use 192.168.128.0/30 and then use 192.168.128.2 for the MX IP address instead of 192.168.128.254. You could keep the interface IP address of 192.168.128.1 on the MS.
On this uplink VLAN defined on the MX, does DHCP need to be enabled for any reason? I don't see why it would, but I'm unsure.
On the MX, how would you define the uplink port to the MS? Trunk port, native VLAN 1 and allowed VLANs 20? The answer to this would dictate how the matching uplink port on the MS would be defined.
The subnet size is not very relevant. Note that with an L3 switch the switch management address (which is what the MS uses to talk to the Meraki cloud), which is not the same as the VLAN address (which is used for routing packets) also needs to be in that uplink range.
That would preclude a /30. A minimum of a /29 would be needed.
I try and use DHCP for the switch management address so that if I need to make config changes (such as a change in DNS server) I can do it without needed to access the switch physically via its management interface (in the event it gets taken offline for some reason).