If I understood correctly, for example, you copy 60s of drum, but select a 35s part of the track and, when pasting, you want to paste only 35s (the amount of the track selected).
If that’s the case, Punch Copy-Paste plugin is for you, from Steve: https://forum.audacityteam.org/t/punch-copy-paste/28906/1
(For doing what you want then, you need “punch copy” and “punch paste” from the plugin, not ctrl+c and ctrl+v. Instructions in the official post there, but easy!)