I usually draft flow diagrams in powerpoint and paste into the word doc as a static image. I find ppt is easier for layout (just don't forget to save the file with your manuscript).
On a side note...
- You've got redundant information in your 'records removed before screening' box. Your top level already says you've removed them, so don't need to repeat that in the individual reasons (i.e. just need 'Duplicate records', 'Other reasons')
- You switch from 'records' to 'reports' halfway through the diagram
