Plug shape question

To start, I have never used a plug with any regularity, but have been into pegging for a bit. For warm up my wife has used our beginner strapon dildo, short and narrow. Outside of the fact that we finally had to throw it away, it never really did what I had hoped we would get from a warm up. She would always start with her hand then move to the beginner dildo then to our current strapon which is only 1.5" wide. She would still have to take it slow when introducing the 1.5" before getting to work. We are stepping up a bit this week but also want to buy a new plug or set along with it. The plan is to insert a plug during the beginning of play to be relaxed and ready when she straps it on.
So, to the question. I know about the size that we are going to want to warm up to but does that need to be at the fattest part or the small section above the base? I kinda like the idea of going a little bigger in the middle and not going too small after getting all of the way in. Problem is that I don't really see the dimensions listed for the tip, the widest part AND the part before the base. I would think that I would need to be at least 1.5" to 1.75" diameter at that point to be more ready for the strapon.
As you may tell, I am uneducated in this particular area, and any help would be appreciated. I have read alot of forum submissions about plugs but none that are specific to my question. Thanks in advance, I know that I can always turn to you guys for help.

I can't offer much help regarding your sizing questions, but I just wanted to ask - have you ever tried a vibrating plug?

I recently bought my first vibrating plug and I can already feel the difference in terms of relaxation when I use it. It feels great while I'm using it, but I also feel more relaxed immediately after I take it out and I'm able to use bigger toys than I normally would.

Vibrations definitey help for me when relaxing.

OP I know exactly what you mean and very rarely do you see the girth measurments for the stem of the plug.

I have just taken to buying lots of different sizes over the years to experiment which have been pretty fun.

The other option is this:

http://www.lovehoney.co.uk/product.cfm?p=15697 which will help a long way to practice for a larger girth.

Alternatively use dildos/vibrators. Obviously you wont be abe to keep them in with assistance but they wil have girth measurements and also get you more ready for the strap on as that sort of penetration is deeper than butt plug penetration which you may also want to get accustomed to.

No I have never tried a vibrating plug. Good idea though. Never even thought about it.
Just Jenson, I like the shape of those. They look like they would stay in and hold a decent size vs a slim stem. Also, we have used vibrators but would prefer something to insert and forget for a bit during other play.