[votan]

<emperor>Welcome on the dark side</emperor>

You are right, size is used as a value, only. By default. But by the power of Lua you can intercept accessing properties like "size" to change the returned value dynamically.
For example:

function addon:HookPOIPins()
    local GetCurrentMapIndex = GetCurrentMapIndex
    local function HookPinSize(data)
        local orgMetaTable = getmetatable(data)
        local orgSize = data.size or 40
        data.size = nil -- Force to ask the metatable
 
        local newMetaTable = {}
        setmetatable(newMetaTable, orgMetaTable)
        local alter = {}
        alter.size = function()
            return GetCurrentMapIndex() == 1 and 1 or orgSize
        end
                -- more alternates here
 
        newMetaTable.__index = function(data, key)
            return alter[key] and alter[key](data) or newMetaTable[key] -- if alternate exists, call it
        end
        newMetaTable.__newindex = function(data, key, value)
            if key == "size" then
                orgSize = value
                return -- Do not set value within table to keep using metatable
            end
            return rawset(data, key, value)
        end
        setmetatable(data, newMetaTable)
    end
    HookPinSize(ZO_MapPin.PIN_DATA[MAP_PIN_TYPE_FAST_TRAVEL_WAYSHRINE])
    HookPinSize(ZO_MapPin.PIN_DATA[MAP_PIN_TYPE_FAST_TRAVEL_WAYSHRINE_CURRENT_LOC])
end

Of course you have to adapt this to your needs.