Not aware of good resources to find the sizing. Typically the information found in workshop manuals not available to everyone.
Try reaching out to Cox Motor Parts or Carbolts

 

Not knowing the business models on your side of the Pond, does the UK have a big-box help-yourself type of hardware stores? Here in our side, big-box stores have templates of various sizes of nuts and bolts. These templates allow you to compare your original nut / bolt to a standard. Knowing the diameter / pitch / length of the bolt in question should make replacement quite easy.

Take your unknown bolt/bolts to one of these stores. Try them out on the template & write down those measurements. Also take note of what ever symbol is stamped on the top of your old bolt. Whatever new bolt you purchase MUST have the same information stamped on it. While some hardware items can be used on vehicles, some may not be suitable for auto use due to the stress / vibrations found in and around the engine compartment.

If in doubt about the hardware store item, take your measurements to an auto supply store. They should be able to match your old bolts to new bolts made to auto standards.

Good luck. Keep us posted as many other forum members participating in DIY procedures may need to replace that one nut / bolt that has dropped down into the inner depths of the engine compartment never to be seen again, but heard every time you go over a bump.