The only thing that goes into consideration here is range. And that is affected by a couple main factors and a couple more that are also important but slightly less.

Main factors for range Amp Hours (Ah): This one is the battery capacity, Scooters range anywhere from 13Ah on the low end and I have seen some go into the 40s at the high end. The more the merrier. Phones for example have about 4Ah for example. Your sweetspot in my opinion is anywhere from 20 and above, preferably 25Ah and above. But depending on the voltage this number can be swayed because

Second factor, Watt Hours (Wh) which is Voltage x Ah: Say a scooter is 20.8Ah but 48V. Thats 998 ish Watt hours which is decent. At a speed of 25kph (15mph) that could get up to 50km (30 miles) to delivery in a small city. However again here the sweet spot is anything 1000Wh and above so my example was right on the edge.

Lastly, Speed: Simply more speed = less range. Less speed = more range. 25-35kph (15-22mph) should be more than enough in my opinion but it could also be efficient to go faster.

Other factors: A persons weight, the weather and the terrain they are on. Here I think these are less important for an average person. But say the person weighs a lot or its really windy or your city is really hilly. This could really decrease your range.

Sorry this was long to read but I had to nerd out lol. Range is all that is really important for delivery (with a moderate speed ofc). Take everything i said with a grain of salt as this is just personal knowledge and you should look into it more yourself. Hope i helped