Hi Pradeep,
I believe we've already been in contact with your company regarding this (they have a support contract), with an initial patch to at least help filter these by substring match.
The repository list had to be capped at a number, to avoid some scalability issues (some companies had many thousands of repositories, making this page almost unusable). We changed it to load and filter via the API back in Review Board 4, and this presents a maximum number of repositories per load. However, filtering only works by prefix-matching, and in a setup where all repositories start with the same handful of characters, that's an issue. The patch we provided should address that, and we're planning a patch to extend the number loaded per page.
We are thinking about improvements to this whole flow, but for the time-being, once the patch is applied, I'm hoping it will ease your workflow.
In general, we recommend using RBTools to post changes for review, rather than the New Review Request page, as RBTools works around a number of issues that can come up with patches, and provides an easier way to update review requests from changes in your tree.
Christian