user13340: I would like to write a story featuring creatures living in a ecosystem not unlike the one we know, but still they are alien creatures. I would like the details of these creatures' functioning to be as scientifically sound as possible.
Would it be ok to ask questions like
Given the size of the animal and their species, how much would they weight?
How much blood would they have?
What should the caloric value of their diet be?

If those kinds of questions would not be appropriate to Biology StackExchange, how could I rephrase them so that they would be?

user3340: The theoretical aspects are fine and people have asked question like this (good theoretical framework for a sci-fi novel etc). However, you cannot go too speculative and ask very broad questions. You can break your question down to specific points such as:

What are the theoretical limits of mass/density of a certain organism at a gravity of 2×g?
How much volume of blood is required by an organism of mass=x (there are other parameters to consider such as partial pressure of oxygen in the atmosphere etc)?

That these all would fall under the topic of theoretical biology and you need to provide sufficient details if you want a hypothetical model. Moreover the answers are likely to be speculative but can be based on good reasoning.
I think this is on topic as long as you provide sufficient preliminary assumptions for your model. 

user69: I'm not sure that these questions would be a good fit for our site - in the past we have always limited ourselves to the real world when presented with questions along the lines of where is the best place to shoot a zombie etc. Whilst your questions may be better thought out, allowing them might blur the lines.
Have you considered the World Building stack exchange?

13340: <p>I would like to write a story featuring creatures living in a ecosystem not unlike the one we know, but still they are alien creatures. I would like the details of these creatures' functioning to be as scientifically sound as possible.</p>

<p>Would it be ok to ask questions like</p>

<pre><code>Given the size of the animal and their species, how much would they weight?
How much blood would they have?
What should the caloric value of their diet be?
</code></pre>

<p>If those kinds of questions would not be appropriate to Biology StackExchange, how could I rephrase them so that they would be?</p>
 3340: <p>The theoretical aspects are fine and people have asked question like this (good theoretical framework for a sci-fi novel etc). However, you cannot go too speculative and ask very broad questions. You can break your question down to specific points such as:</p>

<ul>
<li>What are the theoretical limits of mass/density of a certain organism at a gravity of 2×g?</li>
<li>How much volume of blood is required by an organism of mass=<code>x</code> (there are other parameters to consider such as partial pressure of oxygen in the atmosphere etc)?</li>
</ul>

<p>That these all would fall under the topic of theoretical biology and you need to provide sufficient details if you want a hypothetical model. Moreover the answers are likely to be speculative but can be based on good reasoning.</p>

<p>I think this is on topic as long as you provide sufficient preliminary assumptions for your model. </p>
 69: <p>I'm not sure that these questions would be a good fit for our site - in the past we have always limited ourselves to the real world when presented with <a href="https://biology.stackexchange.com/questions/2774/what-is-a-way-to-kill-zombie-does-my-suggestion-work">questions along the lines of where is the best place to shoot a zombie</a> etc. Whilst your questions may be better thought out, allowing them might blur the lines.</p>

<p>Have you considered the <a href="https://worldbuilding.stackexchange.com/">World Building stack exchange</a>?</p>
