Fleas are small.
Fleas are numerous.
What type of informal fallcy is this?

They are not small.
Define small,has many definitions.

Fleas are small.
Fleas are numerous.
What type of informal fallcy is this?

What you have here are two premises. This in itself doesn't constitute a fallacy for this to be a fallacy you would need to include a conclusion. What we would have then is deductive logic in the form or a syllogism. A syllogism is defined as an argument which contains two premises and a conclusion.

For example

Fleas are numerous
Fleas are small
Therefore fleas bite

The fallacy here is that the conclusion does not follow from the premises. Because fleas are numerous and they are small it does not follow from the premises that they bite.