Most sizing advice counts the people in the house and the laundry, then stops at the back door. If the softener feeds every tap on the property, the outdoor taps and the irrigation system draw on the same tank. That extra draw can be large and uneven, and it changes how often the softener regenerates and how quickly the resin uses up its capacity. Size the unit for the whole property, not just the kitchen sink.
Find out where the outdoor lines connect
Trace the supply before anyone quotes a size. Ask the installer which side of the softener valve the hose bibs and the irrigation mainline connect to. Some layouts send the outdoor lines ahead of the softener, so they get untreated water and never draw on the resin. Others run them after it. Get this answer in writing on the estimate, because it decides whether outdoor use counts toward the softener's capacity at all.
Measure the outdoor draw, not the guess
Ask the irrigation contractor for the zone list, the gallons each zone uses per run if they have that figure, and the run schedule programmed on the controller. Multiply each zone's water per run by the runs per week, then add any pool fill or top-off routine you have. Do this for the driest month you expect, because the sizing has to hold in that month, not the average one. Hose and car-wash use is smaller but adds up, so include it if you do it regularly.
Decide whether you want softened water on the yard
Settling the sizing question is not the same as deciding where softened water should go. Whether softened water is a good choice for lawns and gardens depends on the plants, the soil, and the water source. That is a question for your irrigation contractor or your local extension office, not for the softener salesperson. If you decide the yard should get unsoftened water, ask the installer to price a separate outdoor line ahead of the softener, and get that cost on the estimate as its own line item.
Check the regeneration control against the outdoor load
Ask whether the control is metered or on a timer. A timer-based unit regenerates on a set schedule whether or not the outdoor draw has used the resin, so a heavy summer can run the tank low before the next regeneration, and a quiet winter can waste salt. A metered unit tracks actual use and handles a variable outdoor load better. Ask the installer to show the regeneration setting they plan to program and the reserve you will have during your heaviest irrigation week. Ask what changes in the setting if the irrigation schedule changes later.
Plan for winter and for service access
Outdoor lines that run ahead of or behind the softener are exposed to cold in a different way than the indoor plumbing. Ask who drains the irrigation system for winter and whether the softener's bypass and shutoffs can be reached without shutting off the outdoor line too. If the outdoor run passes through the softener area, confirm that a shutoff exists on each branch so one line can be isolated for service without taking the whole house off the softener.
What to put in writing before you sign
Get four things on the estimate: which side of the valve the outdoor lines connect to, the sizing figure that includes the outdoor draw and the month it was based on, whether the control is metered or timed and how it is programmed, and the price of any separate outdoor line you chose. An installer who sized only from indoor use has not sized the house you are going to live in. If you are comparing firms, the company ratings list the scores for local firms, and the rating methodology explains the criteria behind those scores.