Hi Peggy,
What an interesting project! To make a soap dish in the larger size, you will need a mold that size. Since it sounds like you don’t have a mold already like that, you will need to make one. You will need to have a template, then make a mold of that template. This article details more: https://resinobsession.com/resin-frequently-asked-questions/tips-for-selecting-a-model-for-your-next-mold Once you have your template, this article should help you get started: https://resinobsession.com/molds-mold-making/basic-mold-making-tips
In terms of using Easy Cast resin for this project, you will likely be okay. In my opinion, I think when fully cured, this resin is still rather soft. If anything, I would worry about it denting if someone dropped a casting this size on the floor.