[Banknorris]

Working on the previous solution, maybe:

G["castbar"] = {
    ["player"] = {
        ["width"] = 350,
        ["height"] = 35,
        ["point"] = {"CENTER", UIParent, "CENTER", 250, -200},
        ["iconSize"] = "height",
        ["spacing"] = 2,
    },
}

then when using the configuration do something like:

local height = G["castbar"]["player"]["height"]
local iconSize = G["castbar"]["player"]["iconSize"]
if iconSize=="height" then
        iconSize = height
end
castbar:SetHeight(height)
icon:SetSize(iconSize,iconSize)

If in the future you decide to set iconSize to lets say 50, you just need to change the configuration value from "height" to 50, the code the uses the configuration will still work.