'Which were a size too small.' or 'which size were too small.' Which one is correct?

The whole sentence is

Mr Boxell had deliberately sold the man a pair of shoes which were a size too small, knowing he would return them next day!

I'm so confused about which were a size too small, whether it is the correct order.

He is saying that the shoes were one size too small (e.g. were a size 10 instead of the size 11 that the customer required).

Your other example could also be used, but would have to be whose size was too small, but the wording in the original sentence is more correct.

The usage of "which were a size too small" is correct in this sentence-

Mr Boxell had deliberately sold the man a pair of shoes which were a size too small, knowing he would return them next day!

"A" here indicates one size small.

If you take a look at this lifestyle blog, there is a similar usage of "a size too small"-

Ever go to the store and find a HOT pair of shoes on sale that are just half a size too small?

Nothing seems wrong with word order in 'Which were a size too small.' Substitute the word "one" for "a" to get implied meaning. The second choice which could be written as "which were too small" is less specific. How many sizes too small?