I downloaded an icon pack and there isn't an icon for a certain category in my project.I want to keep my icons consistent with the look so what should I do in this situation? Do I just look for something that is the closest in looks? Also in terms of sizes of the icons, should they be the same width and height? Or is it ok to have one higher than the other?
There are a few options here. I'll list what I usually do below:
If I am looking to create a new icon that matches the set, first thing I'll do it open illustrator to see if I can grab different parts of the original icons in the set to create a new icon. This works fairly well because I know that the weights and structure will be consistent and will look like it was always included in the set.
if option 1 isn't possible, I can usually find the icon in another set that matches pretty well. Most users will not notice the difference if you choose the second set wisely.
If neither of the above options are possible. I will create an icon from scratch using the weights and fills from the original set as a "template", trying to maintain the look and dimensions as I go.
Also in terms of sizes of the icons, should they be the same width and height? Or is it ok to have one higher than the other?
I'm not quite sure what you mean by this one. I'll provide a couple answers based on how I can interpret your question.
if your question is asking about visual use of the icons
Consistency in your design is paramount. Making similar objects visually equal help the user to quickly digest and categorize information. If similar objects (icons) have a different weight or scale, users will perceive the object as being unique to the other objects, which might lead to confusion or a different interpretation of the message being delivered.
if your question is in regards to implementing the icons on the web
Yes you should try to keep the icons similar heights/widths. This makes it easier to implement without having to control the presentation with css.