Points to consider are the tassel length; the tie back needs to look generous and a comparatively small tassel on a large curtain will lose any impact. Cord length is another consideration as a short cord will be too tight for a thick curtain. The weight of a tie back is again relevant to fabric but as before it is really up to individual taste Usually all tie backs are constructed on the large tassel longer cord, small tassel smaller cord ratio.
How do you know where to position tie backs?
Positioning of tie-backs should allow them to fall two thirds down from the top of the curtain and allow the curtains outside edge to remain in a straight vertical - something resembling a reverse capital R but with the “waist’ of the R a little lower. When measuring the position of the hook on the wall to hold the tie back in place, it helps to hold the tie back around the curtain to determine and mark the exact position for the cord hook as proportions of window/wall/curtain etc are always different in each setting and whilst not common it may be that your window needs the tie back placed differently.
The Victorian Emporium sells a great range of beautiful Victorian tie backs and hold backs for every room in your house.