What do you mean by "size"?
Exporting as "Original" just copies the original file to the destination without any changes, exporting as "DPX" makes a sequence of individual frame files each with a DPX extension, and exporting as "H.264" makes a .mp4 file with an H.264 format video track and an AAC format audio track. Only DPX and H.264 allow changes made by your Develop settings copy-pasted to the Library video to be a part of the output.
I can clarify. I have 4K footage. I want to trim it and do a few basic colour changes. I want to export as the 'original size' (4K).
How can I do this?
LR is currently limited to 2K 1920x1080 as it's maximum Export size for modified videos.
If you have a Photography CC subscription for $10/month you can open the video in Photoshop CC 2015.5 and use Adjustment Layers or some Filters to make certain adjustments and render at 4K.