It's been a while since you posted, but I'll take a stab at helping nonetheless. It's tough to give you complete specifics for your exact printer, but here are some basics for printing a customized paper size from InDesign... (I have ID 5.5, so I'm using that to give these instructions, but the concept is the same for CS3 and up).
The "custom" setting is grey or unavailable until you set up a new setting...so let's do that.
Choose the printer you wish to use - in your case, the Epson 1100.
The PPD should fill in automatically, if not, you can choose the appropriate PPD (the print driver for your printer).
Now you need to go through the windows on the left side of the screen to set up the rest of your print options.
Choose the number of copies, page range, etc. If you have two pages in a spread and want to print them together, check the "spreads" box. I think the rest of this page is self-explanatory.
Choose a paper size here. This is where things can get a little tricky and where you'll have to answer a few questions for yourself... What paper size are you actually printing on? It looks like the Epson 1100 prints on 11x17 and 13x19 sizes. Depending on your PPD, these sizes may already be listed in the "Paper Size" drop down menu. If not, you can add the size using the "Page Setup" button on the lower left hand corner of the Print dialog box. (You'll see a warning message pop up, you can ignore it for the moment. In the new window, under "Paper Size" choose "Manage Custom Sizes" and enter the dimensions of the paper. Click OK. Make sure the new size is chosen in the Page Setup and click OK again.) Back in the Print dialog box....now you should have the correct size listed under paper size. If your original document is larger than 11x17 or 13x19 as you indicated, then you want to look under the "Options" section and click on "Scale". Choose between Width (where you can choose exact scaling yourself) and "Scale to Fit" (where InDesign will do the scaling for you). (*I'll note that this is all completely unhelpful if you actually want to print at EXACTLY 12x18 on a larger sheet of paper. If that's the case, might find it faster to export your document in a PDF, resize your pages to 12x18 and then print them out.)
Marks & Bleeds:
If you want to include crop marks, registration marks, etc. this is the place to do it. If you want to include any bleed on your print out, you can do that on this page as well.
Looks like the Espon 1100 is a CMYK printer, so this window will probably say "Composite CMYK". If you wanted to print separations, you may have that option on this page if your PPD allows it.
Images - Send Data - set to "All". You may or may not have other options available depending on your PPD, set them as you feel appropriate. Postscript level 2 or 3 should be fine.
I've found that many people have very specific color management preferences of their own, so set this as you prefer. However, if you don't have any particular preferences, I'd suggest that you set the color handling to "Let InDesign Determine Colors". It's likely that you'll have a default printer profile show up, and it may be best to go with that.
OPI - hopefully this is grayed out. If it's not, make sure that nothing is checked.
Transparency Flattener - If you don't have your own settings, then set this to High Resolution
Now, before you print, click on "Save Preset..." and all of the preferences that you just set up will be available the next time you want to print. The "Custom" tab at the top will have a new preset available with your new settings! Ok, now print!
I hope that helped or helps in the future. Good luck with InDesign and your new Mac!