Frozen Prey...choosing size

I am going to increase Fosters prey size or try to offer her two on her next feeding but I was wondering...what is the best way to guage the proper prey size, by snakes girth or percent of weight?

The reason I ask is because the other day I bought a small rat which was the very, very close in girth size but more than the suggested 10% of total weight, when thawed. The rat weighed almost 60 grams which is about 20% of Foster size. I was scared to death offering it to her so thankfully (in a way) she didn't take it. I've offered two mice in the past but she didn't take or even show interest in the second. I've since found out that I should have offered it right away instead of waiting a few minutes like I did.

Since she didn't take the rat I'm still debating if I'll try a smaller rat again or try to offer two mice. I asked in another thread but didn't quite get an answer (no offense to those who did reply). I have only offered her pure white mice in the past and the rat I got had a black head. What do you suppose was the reason for her not taking it?
1 - Prefers mice over rats
2 - Rat was too big
3 - Didn't like it because it wasn't all white
4 - None of the above
5 - All the above
6 - Other

I know there is no way of really knowing why she didn't take it but thoughts would be great.

Suggestions were given on how to make her think the rat was a mouse but if she was afraid of the black head, or if it was too big, that wouldn't really help.

So, before I go buy more prey and end up getting something I shouldn't should I judge it by girth or percent of body weight?

Something else...because I buy frozen (and this might be a stupid question), if it is suggested to go by a percentage of her body weight, what should I shoot for? I mean, because the prey will be frozen it's naturally going to weigh more.

Sorry for rambling!