I don’t edit with Premiere, but use Adobe Media Encoder. You might be able to do this in Premiere.
Bit rate and resolution are the parameters that most affect file size. I’d start with a bit rate of 500kbps and lower your resolution to 720P. AME will show you the estimated file size after you enter the parameters.
H.265 will give you better quality compared to h.264 at the same bit rate. So you can go to a lower bit rate with h.265 and maintain quality, if the copyright office accepts that CODEC.
Be sure to use 2-pass encoding as that will increase quality compared to 1-pass. Also, use variable bit rate. Hope this helps.