I could not find a 64-bit version of LibreOffice available for Windows on the website, only for Linux. Are there plans to release a 64-bit version for Windows any time soon? Would be useful for large spreadsheets on LibreOffice Calc.
Question now obsolete:
Starting with the Alpha and Beta prereleases and since 5.0,0.5 (2015-07-26) being the first released version of the 5.x family there was a 64 bit build for Win in addition to the 32-bit build. The present support for still versions will surely not include porting to Win 64.
Please take a look to this bug report 64-bit version of LibreOffice on Windows and follow the link in first comment.