OK, for anyone looking to do what I did (or if you fancy implementing it):
Firstly, I created a settings.lua file in the root folder (where data.lua and info.json are). In that file, I included the following:
data:extend
({
{
type = "int-setting",
name = "small_tank_size",
order = "a",
setting_type = "startup",
default_value = 500,
minimum_value = 250,
maximum_value = 10000
},
{
type = "int-setting",
name = "large_tank_size",
order = "b",
setting_type = "startup",
default_value = 2000,
minimum_value = 250,
maximum_value = 10000
}
})
That creates 2 variables. To actually use them, I edited prototypes/entities.lua . That file defines both fluid tanks as fs-50 and fs-200. We need to change the volume of those. They're both a height of 1, so their volume is equal to their base area * 100. The base area for fs-50 is on line 22 for me, which I changed to base_area = settings.startup["small_tank_size"].value, . The base area for fs-200 is on line 141, which I changed to base_area = settings.startup["large_tank_size"].value, .
Finally, we want to make this look pretty, so we need localisation strings. I don't speak Russian so I only added English localisation, but the process is identical for Russian as well. I edited locale/en/locale.cfg . I added the following entries at the end of the file, giving the menu options a localised name and a localised mouse-over tooltip:
[mod-setting-name]
small_tank_size=Small tank capacity
large_tank_size=Large tank capacoty
[mod-setting-description]
small_tank_size=The amount of fluid the small tank can hold. Actual value is multipled by 100. Default is 500.
large_tank_size=The amount of fluid the large tank can hold. Actual value is multipled by 100. Default is 2000.
That's your lot. The first file creates the options, the last file creates human-readable text for the options and the tweak to the middle file changes the tanks from using a fixed size to using whatever size the player enters through mod options. I did spend some time reading up on mod structure, but most of this is cribbed from other mods with options. I really hate Lua as a programming language so this seemed like the easiest approach :)
edit
The text editor on this site eats code tabulation and I don't know how to post a code block. Sorry about that.