Which institution's logo to add to research poster when research was done at institution different to main affiliation as student?
If both institutions have supported your project, you should acknowledge both in your poster.

To save space, you have a variety of options:


Make the logos smaller by scaling them.
If available, use a small, square version of the logo rather than a larger rectangular version.
Put the logos in a part of the poster that is underutilized. (For example, left-align the title and authors, and put the logos over to the right side next to the author list.)