Hello Kris. The size of your layout, whether it is 60'x 90' or 600'x 900' does not determine whether you need a booster or not. Whether you need a booster will depend on the number of locomotives you are going to run at one time on the layout and where your command station/booster is located on the layout.
For information with respect to the number of locomotives, visit this page on my website.
http://www.railwaybob.com/Modules/WhatIsDCC/DCCWhatIs01.htm
On a large layout, the location of your command station/booster on your layout and the size or gauge of wire on your track power buss and feeder wires will determine whether you need to add a booster.
For the location, assume you have a point-to-point layout. As you run a track power buss around the layout to feed the tracks with power, there is a drop in the voltage of the wire. This drop is small on small layouts but can be large on larger layouts. If you were to locate your command station/ booster in the middle of your layout, then the maximum length of your track power buss will be about 45' (45' to the west, 45 ' to the east equals 90'). A length of 45' should not result in a significant drop in power if you are using a heavy gauge wire such as 14 AWG stranded wire.
However, if you place your command station at one end of your layout, the maximum length of your track power buss will be 90'. In this case, you would probably want to add a booster.
Which brings us to the thickness or wire gauge of the wire you are using to bring the power to the tracks and how frequently you bring the power to the tracks (ie the number and location of your track feeder wires. If you are using telephone wire (24 AWG wire), the voltage drop is quite dramatic as you go out each yard. However, if you use 14 AWG wire, the voltage drop isn't as dramatic. Similarly, if you have track feeds every 10' - 12', there is quite a drop in voltage along the length of track that is fed by each track feed. However, if you have track feeds every 3' - 4', the voltage drop isn't as dramatic.
So, if you are using telephone wire to bring the power to a 90' layout, you will definitely need a couple of boosters to boost up the voltage. The solution in this case would be to use some heavy gauge wire - 14 AWG wire as a minimum. (Remember that as the AWG number decreases, the wire thickness increases. That is, 14 awg wire is thicker than 24 AWG wire.) For a discussion on wire gauge and voltage drop, visit this page on my website.
http://www.railwaybob.com/Modules/TrackBuss/TrackBuss01.html
The shape and format of a booster will depend upon the manufacturer of your command station/booster and whether it is compatible with the other boosters that are made by manufacturers. In your case, you are using a Lenz system so Lenz probably has a booster that can be added to their system. I'm not at all familiar with Lenz so I can't help you out specifically.
In the case of Digitrax, to add a booster to the system, you simply buy another command station/ booster and configure it as a booster. For example, I have a Digitrax DCS100 command station/ booster. I could purchase another DCS100 command station/ booster and configure it as a booster, or I could purchase their lower priced DBS150 command station booster and configure it as a booster. Or, I could purchase a lower-priced booster from another manufacturer if it was compatible with my DCS100 command station/ booster.
Perhaps some other people who use Lenz could help out here with the specifics.
Bob M.