if the storage space in new mac is not enough, then how to transfer data from old mac to new mac

Refer to How to move your content to a new Mac.

Under Select the Information to Transfer, you can select among broad categories of information to transfer. For example, click the "reveal triangle" adjacent to a User Account, and de-select Documents.

Per Apple's published return policy:

Returns & Refunds - Shopping Help - Apple

you have 14 days to return the current computer for a full refund and then buy another that has sufficient storage for your needs.

The other option is to "triage" what is on the old computer and decide if less important files can be stored on an external drive. That way you still have access to the old files without over-filing the new computer's storage.

You basically have to decide on how much of that data (on your old Mac) that you want to delete or archive BEFORE you attempt to migrate it to your new one if you are not able to obtain a new Mac with the same or larger internal drive space.