I cut vinyl film for use on different shaped objects like buckets and christmas ornaments. I have seen a tutorial on how to use the warp feature, but my question is how do I size it to fit my specific object? Is there some way to represent my object on the canvas to fit text and / or images to it?

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I don't think Inkscape has a feature that will do what you're describing....or at least what it sounds like to me, that you're descibing. Can you show us the tutorial you found? That will tell us which feature you're talking about. I haven't heard about a warp feature for Inkscape, but that doesn't mean it doesn't exist.

Oh, unless you're using the Eggbot extensions? Possibly they could do that.....well, I'm not entirely sure, since I've never used them.

Anyway, I would need to know more about the warp feature, to be able to help. Or maybe someone else already knows about it??

Here is a link to the warp tutorial on youtube: http://www.youtube.com/watch?v=iCHv34FkLGo

Warping looks like it's what I need to do to make my text and graphics fit around cone and round shaped objects. But I don't know how to put the object I need to fit it to on the canvas, so I can get everything the right size.

Here's an old topic that might relate at least to the bucket. essentially you will have to know - and draw! -the unfolded shape and dimensions of your object on the canvas, because Inkscape will not do that for you. it might be a lot of trial and error.
For the sphere, if you have 3D knowledge you might be able to get what you want from Blender and work with the UV map in Inkscape, but I don't recommend trying to learn Blender just to do this!

Wow thanks it looks like there might be a solution in all that. Trouble is, I don't understand what they're talking about. I'm a novice with graphics programmes. I've been trying to search youtube for a video that might help me.

I think you will just have to measure your objects and create a document of at least that size and carefully create objects of the desired size, watching the control bar etc. for correct dimensions. I don't think 'warp' (that's what the youtube video maker calls the envelope deformation path effect, for all those who don't want to sit through another youtube vid) is going to be much use to you unless you first draw the shape of your object on the page. If you want help with that you will have to provide much more specific info...

Aahhh, Envelope Deformation. Yes, that could be used for that, I guess. For a xmas ornament, I'd say just draw a circle the same size as your ornament. Or if the ornament is not a sphere, draw whatever shape it is. (Although note that an Inkscape inch might not be the same as a real inch. It depends on your screen resolution. Someone posted a link to a place where you can enter your screen res and calculate to "real" size awhile ago. I thought I had saved it, but I can't seem to find it.)

Oorrr....what about making the graphic that is to be warped, the same width as the ornament, before you start to warp it? That seems reasonable to be anyway. Be sure to scale in proportion.

I might be a bit late, but I thought that I would add a couple of suggestions. If you are using text on a cone shape you should use Text on Path, if you are using a vector, then Pattern along path might work well (either single stretched or repeated stretched). There is a nice cone pattern generator that will give you a perfect curve for your cone if you have some simple measurements. Generate your svg then open it in Inkscape and use the Node tool to open the Path and reduce it down to a single curve to use for the Pattern on Path or Text on Path.
http://www.red-bag.com/engintools/calccone_svg.php

If ther is any interest I can go into more detail.