Layout size doesn't matter one iota with a DCC system. If you can run wires to the track, then any DCC system of any size will work.
The limitation is the number of locos you will be running at the SAME TIME, since the current limit of the system determines how many locos you can run at once.
For simple math, assume each loco draws as follows:
Non-sound loco: 0.25 amps
Sound loco: 0.50 amps
So take an NCE PowerCab, with a 1.7 amp capacity. This system will run up to 6 regular locos at the same time, or about 3 sound locos at the same time.
If you run multi-unit lashups (as would be typical with modern diesels), then each loco counts against the capacity.
So THATs your limit. It's also considered good practice to not try to power any given section of track in HO with more than about 5 amps. So if you have a larger layout, you will want to have several 5 amp sections, each one powered by a 5 amp booster.