Looks like the original color was Ivory & Brown. The size is up to you. Very large pieces will need a large board if you want to use them in a game.
Help on Choosing Isle of Lewis Chess Pieces
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I think there is not better repro than the one the British museum offers.
Originals were red but aren't anymore. Pay attention to the shapes of the pieces. For example the chesshouse set using the same pieces for both sides. I wouldn't buy that one.
Hi. I just went to the British museum and also to the National Museum of Scotland and saw the Lewis Chesssmen they have at their shops.
As you can see from the photos in the link you provided above, the one in the British museum is white and red, and they use different pieces for each side.
On the other hand, the one at the National Museum of Scotland is white and black (or maybe more like brown), and use the same pieces for the both sides.
As for the shape and design, I believe both museums offer pieces with great accuracy reproduced from the original.
The taste depends, but I was not really moved by the British museum pieces, although they are probably more factual (red rather than black) and you can enjoy more variety of pieces. On the other hand, I really liked the pieces from the National Museum of Scotland, and I indeed bought one set there at their museum shop, and I am really happy now. It is really hard to tell why I liked the one in the Scotland more. Probably one thing is that I don't really like red pieces, although I know that one side of the Lewis Chessmen originally looked red.
I hope you find a set that you like.
I had a hard time choosing but now I have bought the medium Brittish Museum version and it has been shipped.
Maybe I will buy a set in the future which is a true 1:1 size but for now I wanted a more compact version because you need a very large chessboard for the others and I also prefer a version which has red and white pieces. The premium set from the Brittish Museum looks good but is very expensive.
I like the set from the National Museum from Scotland. It has a 3.5" king instead of the original 4" which makes it easier to place them on a chess board .It also looks good with more marks and age effects then the BM versions which makes them look older. It has however not as great variety of chess pieces.
The version from the Chess Guild looks very detailed and is a true replica in size with different versions of the pieces but I´m not sure since it is not so well known as the BM and NMS chess pieces and because I live in Europe I have to pay import charges from shops in the USA.
I made a video showing my chess set.Check it out: http://youtu.be/-FAFE7p8FHc
Get a 
'Custom' Isle of Lewis Chess Set from Ivan Simberg in Montreal, Canada. There wasn't a maker in the world that I could find that would make this custom Isle of Lewis set for me with the unsual peices the do not exist on the market. He made it for me. I know you will never find a chess set or chess maker like this. https://www.etsy.com/shop/chessets
I want to buy chess pieces but need help choosing.
http://www.regencychess.co.uk/isle-of-lewis-chessmen-325-by-national-museums-scotland-p-554.html
http://www.britishmuseumshoponline.org/invt/cmcr38000/?ref=searchzone
http://www.britishmuseumshoponline.org/invt/cmcp33400/?ref=searchzone
http://www.chesshouse.com/The_Isle_of_Lewis_Chess_Pieces_p/saca102.htm
http://www.ebay.com/itm/Isle-of-Lewis-Chessmen-Chess-Set-Harry-Potter-Walking-Dead-British-1-1-Scale-/291207879232?pt=Games_US&hash=item43cd579a40
http://www.ebay.com/itm/ISLE-OF-LEWIS-CHESS-MEN-HAND-CAST-COLLECTORS-SET-K-3-5-rosewood-706-/221511622396
Anyone have any of these ones? I want the one that looks the most like the original pieces.
Some of them have a chessboard included.
Some are original size but would they be too big for a chessboard?