you will likely find that the part is a composite of several more basic parts.
If so, refine it into those more basic parts.
If a conceptual part is at the right level and still does not look like any of your existing part data, estimate it as the first of a new kind of part. In doing so, compare this part to other product parts in the database to get a feel for their relationships and then make your best intuitive estimate of its size.
Then, after you have completed the development work, measure the size of the new parts and enter these data into your database for use in estimating future work.