Opencart 3.0.3.8 default product options have no dimensions for shipping

Hey all, I'm looking for advice on best method of using the product options properly.

I have set everything up ok, using opencart 3.0.3.8 but options do not ask for dimensions , so the shipping is only based on base size of product, not the option selected.

My options are for different sizes of same product (photo prints), so have options for say 10x8, 16x12 and 20x16 (inches) which is all good but shipping is obviously wrong as there's no way to state each dimension for the options, only the weight.

How have you all worked around this? You'd think dimensions should be there by default but that's not the case.
I have shipping calculated at checkout to give the customers the most accurate costs, especially for larger orders of multiple products.

So current setup means the base size is 10x8, shipping is say £2.00 for large letter. If select 20x16 version it still charges based on large letter (£2) instead of medium parcel (say £6) so losing £4 on every sale of larger products.

Obvious work around is to add the shipping loss to the price charged for larger product but that is useless when customers buy multiple products, they are then being overcharged shipping.

Hope that's enough info guys, if not I'll update. I'm assuming default setup of options simply wont do it, so an extension may be needed. Any recommendations of ones you have used? I know there's loads out there but don't want to go buying 1 until I find which actually works properly.

Why have options with different weight but zero dimensions? Just seams odd or half done.

Thanks for your time all.
*also running Journal 3 theme, (sorry to the haters) though I know this does not affect the options available.

You don't state which shipping extension you are using.

One thing we have done in the past is use dimension weights for the product weight and option weights and then use weight based shipping extension. Whether this would work for you depends on your products and shipping types.

Thank you for your reply. So sorry, I knew I'd miss something out, kinda gave a wee hint with large letter and medium parcel but could still of been anything eh, using the built in Royal Mail shipping (UK based)

Yeah that's a good shout with the weight based shipping instead. The main issue I face, with Royal Mail, the weight is not an issue, it's the dimensions. If I crank the weight up high enough, it should then resemble correct cost though buying more than 2 would be double, 3 treble and so on. It would kill the large orders.

I'll give it a bash and see how I get on.

as a side note, I do have (not installed) an extension called "Options combinations - Chained options" that I used in a previous life I remember that does account for dimensions for each option though not sure if it would work without a combination. I just need a simple option with dimensions - not combinations. Might work I 'spose?

Thank you for taking the time to respond, I'll give your suggestion a try.