I found one post where someone else complained about the somehow enlarged clock not letting them resize the taskbar, but it didn't show what if anything fixed it. You might try turning off the clock display entirely, then try resizing the taskbar, and then assuming you wanted to, turn the clock display back on. Right-click the taskbar, click Taskbar settings, then click Turn system icons on or off to turn the clock display off. There doesn't seem to be a way to alter the clock display itself, other than if you right click the clock, then click Adjust date/time, and then click date, time, & regional formatting, though if something's stuck changing something there *might* force a reset. Most of the posts I found wanted to increase the clock size, and T-Clock was often recommended. If you can't get the taskbar to normal size with win10's clock displayed, maybe using T-Clock instead would work?
addictivetips[.]com/windows-tips/customize-the-system-tray-clock-windows-10/