Whether the speaker has a say in the arrangement of a room is entirely down to the circumstances and the wishes of the hosts. You might be lucky enough to be given a say in the arrangement of the room. If this happens, think very carefully about it.
If you are doing a presentation with slides on a screen, you might feel that having your audience sitting in straight lines directly in front of you is best but, if the group is a small one and you are using a flip chart, marker or chalk board or simply speaking with no visual aids, it might be best to have them sitting in a semi-circle around you. This is also helpful if you have objects to show or pass around. You might like a table or desk in front of you or you might think that it would distance you from your audience and so, if you have things that need to be placed on something, a table could be placed to one side.
The arrangement might be determined by the size and shape of the room. Inquire about it and, if possible, visit it beforehand and plan the layout you prefer. Always consider how best to reach your audience. You might have reason to walk around demonstrating things or, if you have set them tasks such as craft activities, you might like to go round and help individuals. In this case, it is a good idea to make sure that there is sufficient room between them. You do not want to find yourself in a situation where the person who most needs help is in the middle of a middle row and is unreachable.
Think about how you are conducting the whole talk and of your expectations for your audience. Will they need to move about to share ideas? Do you want to have brief group discussions? You need to position chairs and tables so that any required movement is facilitated and does not take a long time to complete. Plan how you want the groups to sit and the optimum number in each. You might have to be flexible but you should have a basic Plan A and stick to it, if possible, with a Plan B in the back of your mind, just in case.
Furniture can be a nightmare. a wobbly table that is holding some valuable object, such as something precious from your travels or a projector, can be worrying and distracting. Make sure all the furniture, chairs, tables, charts, easels etc is safe for use before you begin. It can, not only distract, but can be dangerous. A flip chart, for instance is tall and heavy and can do some damage or injury. Make sure all is secure!
You cannot choose the furniture in a strange venue but you can make the best use of it. Make it work for you, not detract from your talk. It just takes some forethought and common sense.