hi..aspect ratio is the ratio between longer dimension to shorter dimension of the element. Based on the detailed investigation by various researcher, it is recommended to use aspect ratio 1 for getting the solution to converge with exact solution. i recommend you to read any text book on finite element method. you can read the book authored by Daryl L. Logan titled 'A first course in the finite element method' , 2007, published by Thomson Canada Limited..ok all the best..

Do a web search on "triangle area" "determinant" and you will find that the area of a triangle (your element) can be expressed as a determinant. Ultimately, the relationship between the vertices and the basis function depends on the geometry, as illustrated in the case of area. If the elements contain acute angles (far from isosceles 60-60-60) you will build the global matrix from many nearly singular (ill conditioned) contributions, resulting in an undesirable outcome, more subject to round off error and associated complications. It is a matter of degree. More elements with more acute angles mean more combined undesirable effects and less accuracy representing the governing equations.