If you want to use Nextcloud for Dropbox is that possible?
Suppose files to be saved are 25GB in size?
Current Dropbox account no larger then16gb (in which we collaborate with other people who have larger Dropbox)
Can you create a Nextcloud of 100gb and have the collaborating Dropbox account synced in it, or can you never store more than 16gb Dropbox space?
In Nextcloud I have more storage space, would be nice if Nextcloud with more space can sync Dropbox with less space,
Unfortunately, I did not understand your problem. The only thing that is certain is that if people used Dropbox before and now you want to build something on it like Nextcloud, Google Drive or Microsoft 365, then the users will always be dissatisfied. Your only choice is to either stay with Dropbox or switch completely to Nextcloud, Google Drive or Microsoft 365. Everything else is nonsense.
Nextcloud is really great. But Nextcloud combined with Dropbox is just terrible. But would also be the case with Dropbox combined with Google Drive or Microsoft 365. Interfaces between different cloud platforms have never been great. Only managers claim that this is quite possible.
Pay more money for more data at Dropbox. That is your solution.
https://www.dropbox.com/plans (e.g. 10 euro for 2 TB a month)
Or change full to Nextcloud.
Managed Nextcloud e.g. 5 euro for 500 GB or 10 euro for 1 TB a month e.g. in Germany.
Advantage: you can change the Nextcloud provider if needed without changing the software 
I would say use Dropbox integration App to connect to your small Dropbox from the Nextcloud and Import all the data from it. Than you can use Nextcloud with a lot of storage and remove all information from the Dropbox.
Working via Nextcloud with Dropbox as mentioned above is not satisfying and not recommended at all.
It can. Just enable dropbox as external storage in your Nextcloud. Details in the appstore or with a quick search for documentation online.
@just
Do you think External Storage backend Dropbox? It is only supported till Nextcloud 19. Maybe you can use WebDAV to access Dropbox directly. Dropbox integration only migrates files from Dropbox to Nextcloud.
I suppose. Zero interest in it myself.
Add Dropbox support to your external storage and mount according to their documentation.