I'm playing on version 1.20.1, with mod version 2.0.77.
In my game, I receive a warning:
"Machine is too large, it may be at most 5 blocks in the x dimension".
Meanwhile, in my common.toml configuration file:

#Define how Reactors works
[common.reactor]
#The maximum valid size of a Reactor in the Y dimension, in blocks.
#Lower this if your server's players are building ginormous Reactors.
#Bigger Y sizes have far less performance impact than X/Z sizes.
#Range: 3 ~ 256
maxReactorHeight = 256
#The maximum valid size of a Reactor in the X/Z plane, in blocks.
#Lower this if your server's players are building ginormous Reactors.
#Range: 3 ~ 256
maxReactorSize = 256
#A multiplier for balancing Reactor power production. Stacks with powerProductionMultiplier.
#Range: 0.5 ~ 100.0
reactorPowerProductionMultiplier = 1.0

A Basic Reactor has a fixed max size of 5x5x5.