1) How should we know the user is given correct values for these
fields ,if he given wrong values what should happens?
When creating a Parcel object in Shippo, you would be the one providing the values for the Parcel dimensions (unless you use one of Shippo's templates). So the only way to know that you have correct values is to check against whatever the dimensions of your shipment are.
2)If suppose I want shipment car or bike then how can ask these fields
from user? Is it required to send these fields for all items?
When getting the different values for the Parcel object, you simply want to provide what the final dimension of the entire shipment would be that you're giving to your carrier. If its multiple boxes, you'd want to look at tutorials around Multi-Piece Shipments.
You can also see more in the references of the API what values you should be providing when using the Parcels endpoint.
Also, you can usually provide your Parcel dimensions inline with your requests for the parcels
field.