There are a few things that you can do to make larger suppositories than those sold in the stores under the brand name Fleet.
A couple of neat things about Fleet Suppositories is that they are readily available in quantities of 100 or so and they melt readily in a microwave in a glass bowel or measuring cup(start with 5-10 sec on high power) and re-solidify just as readily (an ice bath to initially set the shape). This makes them ideal for remolding into any shape you might want. Here's some ideas for the "crafty" amongst you.
I have used several common items for making larger ones:
1) Latex glove - Pouring into the fingers and thumbs makes for some very nice sized suppositories,they are per shaped and you can get the size you want by changing the glove size.
2) A piece of PVC Pipe - with a temporary cap over one end will allow you to cast as big a diameter and length as anyone might be interested in. Once cast and solidified I just remove the temporary cap and push them through the pipe with anything that fits inside the pipe. Easy. Smooth the ends and you have your suppository.
3) if you are interested in larger suppositories, it generally takes 50 or so to make a "Condom" sized suppository but it's a nice insertable. I generally use two condoms as some don't hold up to the heat of the glycerin when melted. These form very nicely and I've even had the "ridges" transfer to the suppository form the condom. You can either make them long and leave some of the suppository extend out of your anus or make them shorter and insert them completely in the rectum. I prefer the fully insertable. You can always feel them and yet you can walk around with them inside you. At this size they tend to act slowly so you can retain them for a long period of time. I've had one inserted in me and then be required to take care of the needs of my partner for a couple hours while retaining my suppository. They are also nice as something to go to dinner with. When they are expelled they can be recaptured, rinsed in hot water (melting the surface and renewing it and stored for future use).
I have experimented with Liquid Glycerin, Peppermint Castile Soap and water added to the melted glycerin to decrease the melt point and make the larger ones melt more rapidly with some success.
Hope this gives you some ideas.
Jag