adding dimensions?

My weight-based setup is working pretty well, except where a product exceeds certain dimensions. I tried giving those items a heavier weight to push them into the right postage rate, but this only works where a single product is ordered, otherwise the weight multiplies rapidly!

Is there any workaround, or way to add rules regarding dimensions, or do I need another plugin to replace this with? If so, can you recommend a simple(!) plugin, ideally free?

Many thanks

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