Preselect value within select, while using v-for, and v-model

So I’m trying to pre-select an option within a select - if it exists.
(The code below is a select dropdown for a clothing product)

However I don’t quite understand how I do this - some advice suggests that I manipulate the v-model ( selectedSize) during a created method but in this context of this code selectSize is a price value (once selected) and if i manipulate the v-model at created it will change all instances of the v-model

I’m trying to write something like (below) on the option itself but this isn’t working.

            :selected="size.size === 'One Size'? true : false"

If the correct method to pre-select an option is to alter the v-model value, how do I do this in this context and with only impacting this instance? The v-model in this instance never gets the 'One Size" property I require

Is there another way to pre-select an option?

    <template v-if="oneSelect">
                    <option value="-1" disabled>
                        Select Size
                        v-for="(size, index) in selectedColorOptions.sizes"
                        :disabled="size.stockLevelStatus === 'outOfStock'"
                        :value="{price: size.formattedValue, index: index}"
                        {{ size.size }} {{ size.stockLevelStatus | readableStockStatus }}
                        <template v-if="isVariablePrice">
                            {{ size.formattedValue | prefixValue }}

    data() {
                return {,
                    selectedSize: -1,