If operating in the UK we would actually advise only specifying ‘package_type’ and not ‘transport_type’ as this may restrict the number of available couriers in certain areas and possibly impact your overall experience.
Regarding package sizes you can see this post on package type mappings, as this will detail package dimensions and their associated size categories.
Once package type is specified, with all other valid job parameters filled out, an appropriate courier can be assigned.
In terms of pricing:
The Stuart API has a feature that will give you a pricing quote based on the parameters of the job you create. By calling the get pricing endpoint you will be able to receive a quote that will be the maximum possible price of the job, based on delivery distance, package size and transport type; We have a guide here that explains this in more detail.
I hope this helps! If you have any further questions please do not hesitate to reach out.