Paul's height is reported to be around 178cm and as shoe size has a rough relationship to height (I read Sherlock Holmes novels as boy and science has supported that since) that height would suggest a size 44 European, 10 US, 9.5 UK.
However remember that Paul doesn't wear shoes manufactured with animal products so your choices will be limited - even restricted and pricey.
If you do follow this plan up, just don't build your child's hopes up that the plan might lead to a meeting with Paul, prepare the shoes with a covering letter explaining the Abbey Road cover link, maybe some drawings - perhaps a copy of the Abbey Road cover? - if you are comfortable perhaps a photo of your daughter holding the shoes and pictures, and package them loosely so security can examine the package without ripping the wrapping. Then encourage her to expect to hand them to a member of staff to be taken back-stage.
Her dream will be realised without the disappointment of not being able to hand them to Paul herself.
At any Paul concert there are many like-minded individuals with similar inventive ideas to grab a moment of Paul's time, a few will succeed but the majority will not, so take care not to disappoint your daughter.