Yes, you can use any size figures in Sword & Spear. Figures are grouped into units which must be equal frontage. There is no figure removal. If you have indiviually based 28mm figures you could use sabot bases, movement trays or just pick a frontage and group your figures into units of that frontage. For example, if you have foot based on 20mm square bases and cavalry based on 25mm frontage, you could use 100mm frontage units, with 4 cavalry or 10 heavy foot (5 wide, two deep) making a unit. For medium and light foot, you would probably want to cut out some 100mm by 50mm rectangles from card and place or stick the figures onto them.
Here are some links to battle reports with 28mm figures:
https://nickthelemming.wordpress.com/2015/04/20/sword-and-spear-photomontage-time/ - and lots more on Nick's blog.
http://polkovnikproductions.freeforums.org/crusade-period-28mm-game-t231.html
http://polkovnikproductions.freeforums.org/a-28mm-dark-age-game-t261.html
Here are some of my 28mm Normans, on sabot bases, facing some more Norman knights based as a unit - with some very good combat dice for my Norman spearmen:
Full report of this game:
http://polkovnikproductions.freeforums.org/normans-vs-scots-aar-t43.html
Here are links to some reviews of the rules:
http://polkovnik.moonfruit.com/sword-spear-reviews/
Mark (the author)